Baker's Note: This cake is incredibly moist and rich, thanks to the generous layers of Greek yogurt frosting, blackberry jam, and lemon curd. In Norway, we call this kind of cake "bløtkake," which means "wet cake." We love moistening our cakes with cream-based fillings, making them deliciously rich. If you prefer your cake a bit on the drier side or not too moist, feel free to adjust the amount of filling. You don't have to use all the filling in the recipe—just eyeball it to your preference. For this cake, I did not use all of the fillings and kept about one-eighth on the side. Keep in mind that the more filling you use, the moister the cake will be. If you’re not a fan of a very moist cake, you might want to use less filling.

To better synchronize the downtime in all these recipes, start making the lemon curd first, then the blackberry jam. While these are resting in the fridge, then start making the cake layers.
 

Fluffy Vanilla Sponge Cake
 

• When preheating the oven for my cakes, always use the 'top and bottom heat' setting. Please avoid using the fan-forced option, unless otherwise specified.
   
• Twenty minutes or longer before baking, set an oven rack in the center of the oven, and preheat the oven to 170C°/ 338F°. Using an oven thermometer will ensure even baking results each time, as every oven is different. Always go with the reading on your oven thermometer rather than the temperature you set on your oven.
   
• Grease your wire racks lightly with butter and set them aside; this step will prevent the cake from sticking to the wire rack.
   
• Use a small piece of cold butter, and gently rub the cold butter over the bottom and the sides of three 20 cm ( 8-inches) round baking pans. This technique of using a small piece of cold butter ensures an even, ultra-thin coating. Place a circle of parchment in the bottom of the pan. Add a tablespoon of all-purpose flour and tap it around the inside of the pan until all the buttered surface is covered 100%, then turn the pan upside down over a sink and tap out the excess flour. If you don’t have three pans, it’s okay to bake the cakes in stages; The cake batter will be kept at room temperature until needed.
   
• Bakers note: The egg white to yolk ratio in an egg can vary to such a degree. Therefore, weighing the egg yolks and egg whites as mentioned in the recipe and adding or reducing them if needed is advisable. Place 7 large eggs in lukewarm water for a few minutes.
   
• Bakers Note: All-purpose flour with 8-9% gluten is ideal for tender sponge cake layers; I use 9%. While 10% works, less gluten is preferable. Enhance tenderness by mixing in potato starch which is labeled as potato flour in Scandinavia.
   
• In a medium bowl, sift together 90 grams of all-purpose flour or pastry flour, 90 grams of potato starch, and one teaspoon of baking powder. To get the cake you truly deserve, sift the flour mixture twice to make the flour as aerated and light as possible.
   
• For the yolk mixture, in the bowl of a stand mixer fitted with the whisk attachment, whip 50 grams of superfine sugar, 140 grams of egg yolks, and two teaspoons of vanilla bean paste on medium speed until pale yellow, tripling its volume, and all of the sugar is dissolved—about 5 to 8 minutes. Set aside for now.
   
• Wipe a stainless steel bowl and the whisk of an electric mixer with a paper towel dampened with a little lemon juice to eliminate any trace of grease.
   
• Put 210 gram egg whites and a pinch of cream of tartar into the bowl. Using the whisk attachment, whip on medium high speed for about 3 minutes until the whites are glossy white. Add the sugar ( 200 grams) one tablespoon at a time over 3 minutes to make a very shiny and stiff meringue. Remove the bowl from the mixer.
   
• As soon as the meringue is done, place the whipped meringue into a large bowl, add the whipped egg yolk mixture on top, and carefully fold them together using a balloon whisk rather than a spatula. You don't want to deflate the mixture, so try to do as little folding as possible.
   
• Sift the flour/starch mixture onto the egg mixture in two increments. For maximum volume, use a balloon whisk or a large silicone spatula, and gently yet confidently fold the flour into the batter with strength! It’s better to make a few powerful folds than 15 half-hearted ones. If you don’t get it mixed in just right, you might end up with a sponge cake that’s all heavy at the bottom and too fluffy at the top. You’ll know you’ve done it right when the batter gets a bit less puffy and looks more uniform.
   
•  Use a large silicone spatula at the end of the folding to reach the bottom of the bowl and scrape the batter into the pan(s).
   
• Evenly distribute the batter among the three prepared pans, about 250 grams in each baking pan. Use your offset spatula to spread it into an even layer.
   
• Bakers Note: Give your pans just a really good spin on the table, what that does is it takes the batter up the side of the pan a little bit, just so that as it's baking, it already has an attachment to kind of move up the side of the pan. Don’t tap your baking pans on the kitchen counter to try to get things to level out, especially with this style of cake batter, because banging your pans or dropping it on the table to level the batter actually gets rid of all the air bubbles that you just incorporated into the batter. 
   
• Bake both cakes until golden brown and the top of the cake feels firm and has a pale golden colour. about 15 to 17 minutes, depending on your oven and baking pan. Mine was done at 16 minutes. To check if the sponge cake is ready after 15 minutes of baking, you should first look through the oven window rather than open the oven door. If the center appears slightly sunken, give it a minute or two longer while keeping an eye on it. Then carefully open the oven and gently press the top of the cake. It should feel firm to the touch and may have a slight bounce to it. This method allows you to monitor the cake's progress without disrupting the baking process. Because this sponge cake is going to be filled  with a cream based filling, it doesn't matter if you overbake it for a minute longer, It's better to do that than end up with a sponge cake that is wet in the bottom. 
   
• Immediately without delay run a knife or metal spatula between the sides of the pan and the cake, and invert the cakes onto a prepared wire rack, remove the parchment paper, and leave them to cool completely on a wire rack.
   
• Repeat with the final layer.
   
• If not used immediately, wrap them well in plastic to keep them from drying out.
   
• Each cake layer is about 4 cm high.

Lemon Syrup

• Bakers Note: I moistened the cake layers with lemon syrup because I love the taste of lemon and a slightly moist cake. If you're not a fan of a very moist cake, you can skip this step. The Greek yogurt frosting will also moisten the cake, so it's still going to be moist.
   
• In a small saucepan over low heat, stir together the 70 grams of freshly squeezed lemon juice and 45 grams of superfine sugar until the sugar has dissolved.
   
• Bakers Note: The longer the syrup simmers, the more it will thicken. For this cake, you want a syrup that really soaks into the cake, so try to not have it on the heat longer than needed.
   

Blackberry & Lemon Jam
 

• In a medium-sized saucepan, combine 250-gram frozen blackberries, finely grated lemon zest from 1 lemon, 3 teaspoons cornstarch, 1 tablespoon sugar, and 30 grams freshly pressed lemon juice.
   
• Heat the mixture on medium-low heat until the blackberries are soft and the jam has thicken slighly.  About 5-7 minutes. Make sure to stir at times.
   
• Cool the blackberry jam at room temp, then refrigerate for at least 1 hour or until cool to the touch. The blackberry jam will thicken as it sits in the fridge.

Whipped Greek Yogurt & Frosting 
 

• Bakers Note: Use only heavy cream (or heavy whipping cream) with a 40% fat content. This higher fat content is key for whipping up this Greek Yogurt Cream  that's both thick and stable, providing the perfect base for your frosting. Ensure the cream is very cold for optimal whipping; placing the cream in the freezer for about 15 minutes before use can enhance the whipping process.
   
• Make sure to use full-fat, natural Greek-style yogurt with a 10% fat content. The thick consistency of Greek yogurt, achieved by straining out the whey, makes it an ideal choice for creating a rich and creamy Whipped Yogurt Frosting. Avoid low-fat or fat-free yogurts, as their higher water content can result in a frosting that's too runny for your liking. Your Greek Yogurt should be notably thick. If you find the yogurt too watery, a simple fix involves straining it through a fine-mesh sieve over a bowl, covered with plastic wrap, and left to drain in the fridge for a few hours or, ideally, overnight. This step is crucial to ensure your Frosting achieves the perfect consistency, avoiding a watery outcome that could compromise your frosting's structure.
   
• In a stand mixer fitted with the whisk attachment, beat together 100-gram powdered sugar, 350-gram cold Greek yogurt, 350-gram cold heavy cream, 1,5 teaspoon of vanilla bean paste.
   
• Mix on medium-high speed until light, fluffy and thick. It takes about 1-3 minutes. Whipped yogurt frosting destabilizes (softens) when overmixed, so keep an eye on the frosting as soon as it thickens.
   
• Stop the mixer and scrape down the sides and bottom of the bowl. Then, remove the bowl from the mixer. This should yield approximately 800-850 grams of whipped Greek Yogurt frosting

Blackberry Filling

• Bakers Note: Some deflation is expected, especially when you combine it with the blackberry jam. It’s important to use 40% fat content cream when making this filling; otherwise, it will deflate even more. If you didn't use 40% but let say 35% heavy cream, you can add a tablespoons of blackberry  jam directly on top of the Greek yogurt inside the cake instead.
   
• Remove 400 grams of the whipped Greek yogurt frosting and divide it into two portions of 200 grams each. Retrieve the blackberry jam from the refrigerator. It should have a thick consistency at this stage.
   
• For the first layer, combine 200 grams of whipped Greek yogurt frosting with 100 grams of cold blackberry jam. And for the second layer, combine 200 grams of whipped Greek yogurt frosting with another 100 grams of cold blackberry jam. Cover it with plastic wrap and refrigerate. This will keep it fresh until needed.
   
• Use the remaining whipped Greek yogurt frosting (approximately 400-450 grams) to pipe a border around the edge of the first two cake layers. Cover it with plastic wrap and refrigerate. This will keep it fresh until needed.
   

lemon curd
 

• If you add lemon zest to the lemon curd, be sure to scrub the lemons with a bit of liquid dishwashing detergent, rinse well, and dry before zesting. I use a Microplane zester to collect all the wonderful lemon zest. Before juicing the lemons, please give them a good roll on the work surface to release as much juice as possible.
   
• In a large bowl or your blender, add 90 gram powdered sugar, 17-gram cornstarch, two large eggs, one egg yolk, zest from one lemon ( optional), 125 gram freshly squeezed lemon juice, whisk it all together into a homogenous mixture. If using a blender, process the mixture for 15 to 20 seconds.
   
• Pour the content of the bowl or blender into a medium-size saucepan and cook over medium-low heat, constantly stirring with a whisk or spatula and scraping the sides of the pan as needed, until thickened to a consistency resembling pudding. This step takes about 5 to 7 minutes.
   
• Do not let it boil, or it will curdle. If steam appears, briefly remove the pan briefly from the heat, constantly stirring to keep the mixture from boiling.
   
• When the lemon curd has thickened, like pudding, remove from heat, add in 150-gram cold heavy cream and stir to combine.
   
• This step is optional. I love to blend the finished lemon curd in my blender, Especially when using lemon zest. Just pour the lemon curd into the blender and process/pulse for 5 seconds to emulsify the lemon curd. 
   
• Transfer the lemon curd to an airtight jar and refrigerate until no longer warm—about 2 hours.
   
• One recipe makes about 360 grams; You will have  leftovers.
   
• The Lemon curd will keep refrigerated in an airtight container for two weeks. (Longer storage dulls the fresh citrus flavor.)

Assemble the cake
 

• Baker's Note: Ideally, assemble this cake using an adjustable cake ring with an acetate cake collar to help maintain its shape and ensure even layers. Using this setup from the beginning simplifies the process and provides a clear view of each layer as it's assembled. If you have an adjustable cake ring, definitely use it for best results.

• Place the first sponge cake layer, top side down, on a cake stand that fits in your fridge. Alternatively, you can use a baking sheet lined with parchment paper or a silicone mat, or inside an adjustable cake ring. Just make sure whichever option you choose fits comfortably in your fridge.
   
• Pipe a border of the plain Greek yogurt frosting around the edge of the cake.
   
• Scoop the first mixture of blackberry jam and whipped yogurt frosting (200 grams of whipped yogurt frosting combined with 100 grams of blackberry jam) and fill the middle of the cake with it. Use an offset spatula to spread the mixture into an even layer. Place the next cake layer on top, then gently press down with your hands to make sure it sticks.
   
• Pipe a border of the plain Greek yogurt frosting around the edge of the second cake layer. Fill the center of the cake with lemon curd (I use about 5 tablespoons). Top with 75 grams of fresh blackberries or use the remaining blackberry jam.
   
• Scoop the second mixture of blackberry jam and whipped yogurt frosting (It is best to not add all of it) and add it on top of the blackberries/lemon curd. Use an offset spatula to spread the mixture into an even layer. Place the third cake layer on top, then gently press down with your hands to make sure it sticks. This will help to seal and secure the layers together.
   
• Bakers Note: when covering the cake in plastic wrap, I place a light weight, like a very thin book, on top to help the layers hold together. If you prefer a fluffier cake, use a lighter weight or skip this step altogether.
   
• Cover the entire cake with plastic wrap and place an adjustable cake ring around the cake and refrigerate it for at least 6 hours. Place a small, not-too-heavy book on top. This helps the layers settle and align better.
   
• Once the cake has set, make the marshmallow fluff or any other frosting you prefer instead.

Marshmallow Frosting
 

• Pour about 4 cm (2 inches) of water into a medium saucepan and place over medium heat.
   
• Wipe your mixer bowl, whisk attachment, and hand whisks with a paper towel dampened with a little lemon juice to eliminate any trace of grease.
   
• Separate the egg whites from the yolks. For best results, use fresh egg whites (boxed whites often don’t whip up well). Cleanly separate the egg whites from the yolks, as any bit of yolk can prevent the whites from whipping up. I like to crack the egg on a flat surface, open it up into a small bowl, and then fish out the yolk with a clean hand. This is best done when the eggs are cold, but whites whip up best when they are at room temperature.
   
• Place 120 grams of egg whites, 250 grams of superfine sugar, 2 tablespoons of glucose or light corn syrup, 1 tablespoon of water (the water makes the whipped meringue light and airy), and a pinch of cream of tartar into a perfectly clean bowl of your stand mixer. Whisk the ingredients together to make a thick slurry.
   
• Place the bowl over the pan of simmering water (make sure the pan is small enough that the bowl sits above the water and is not directly touching it, or you may end up with scrambled egg whites).
   
• Whisk constantly but gently until the mixture reaches 54°C/129°F (about 7 minutes). Be sure your sugar-egg white mixture is hot enough to dissolve the sugar, or you will end up with grainy frosting.
   
• Once all the sugar has melted, transfer the bowl to the mixer fitted with a whisk attachment.
   
• Whip on low speed for a minute, then whip on medium-high speed for 4 to 5 minutes, or until the mixture has tripled in volume, is fluffy and glossy, and looks like shaving cream.
   
• Add seeds from 1/2 vanilla bean or 1 teaspoon vanilla bean paste at the very end if using of beating and whip on low speed until just combined.
   
• Make sure not to over-whip the marshmallow frosting. It can get pretty thick and stiff if you overmix it, making it difficult to spread on the cake.

Decorate the cake

• Remove the cake from the fridge. Remove the plastic wrap, the cake ring, and the acetate cake collar.
   
•  Scoop large dollops of marshmallow frosting  on top of the cake.
   
•  Use an offset spatula to apply the marshmallow frosting on top of the cake, working from the top of the cake down. Use the back of a spoon to make pretty marshmallow swirls or just the offset spatula as I do. 
   
• Once the whole cake is covered in marshmallow frosting, Use a kitchen blowtorch, to carefully brown the marshmallow fluff on the top and the side of the cake, Keep the torch moving so it doesn't burn the meringue.
   
• Serve the cake immediately or store it in the refrigerator for up to 24 hours.