Sponge Cake Layers

• 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 two 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 two 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. 
   
• 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 375 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 18 to 20 minutes, depending on your oven and baking pan. Mine was done at 20 minutes. To check if the sponge cake is ready after 18 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.
   
• If not used immediately, wrap them well in plastic to keep them from drying out.
   

Juicy Blackberries
 

• In a  medium saucepan, combine the 250 grams of frozen blackberries, 1 teaspoon of finely grated zest, 2 tablespoons of sugar, and two teaspoons of fresh lemon juice.
   
• Set over medium-low heat and simmer to dissolve the sugar, then bring to a boil and simmer until the blackberries are soft about 10 minutes.
   
• Place the juicy blackberries in the fridge to get cold.
   
• Just before using, strain the juicy blackberries through a fine-mesh sieve to remove most of the blackberry juice.
   
• Save the blackberry juice to make homemade berry juice; just add sparkle water or regular water to your taste. It is delicious.

Blackberry Yogurt Frosting

• Baker's Note: Use only heavy cream (or heavy whipping cream) with a 40% fat content. This higher fat content is key for whipping up 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.
   
• Opt for 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 Greek 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 the bowl of your stand mixer fitted with the whisk attachment, add 50 grams of powdered sugar, 300 grams of cold Greek yogurt (10% fat), finely grated lemon zest from 1 lemon, 300 grams of cold heavy whipping cream (40% fat), one teaspoon of vanilla bean paste, or seeds scraped from 1/2 vanilla bean. Whip the mixture at low speed for one minute, then turn the mixer to medium-high speed and continue whipping the yogurt frosting until light and fluffy, about 1 to 3 minutes. Whipped yogurt frosting destabilizes (softens) when overmixed, so keep an eye on it as soon as it thickens.
   
• Make sure to collect all of the lemon zest from the whisk attachment as it often settles there. Use a spatula to scrape the side and the bottom of the bowl.
   
• Remove the cold juicy blackberry from the refrigerator.
   
• Add the cold juicy blackberries ( do not add the juice) and combine lightly with a spatula. Place in the fridge until needed.

Assembling

• Place the first cake layer on a cake stand or a baking sheet (place parchment paper or a baking mat underneath), and make sure the cake stand or baking sheet fits into your fridge.
   
• Drizzle the first cake layer with passionfruit juice ( if using). Let it soak in the cake layer for about a minute before proceeding.
   
• Add all of the blackberry Yogurt Frosting on top of the first cake layer; with an offset spatula, spread it into an even layer.
   
• Place the second cake layer on top of the blackberry Yogurt Frosting, and press down lightly to help it stick to the filling.
   
• Cover the entire cake with plastic wrap. The cake needs to chill/ rest in the fridge for at least 4 hours, preferly overnight.

Passion fruit 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 a 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 Passion Fruit Yogurt Frosting achieves the perfect consistency, avoiding a watery outcome that could compromise your frosting's structure
   
• Cut the passion fruits in half and squeeze out the inside of 2 to 3 passion fruits. You need 30 grams of both the seeds, juice, and pulp. Place the bowl with passion fruit puree in the fridge to get cold while you prepare the yogurt whipped cream.
   
• In a stand mixer fitted with the whisk attachment, beat together 50-gram powdered sugar, 150 -gram cold Greek yogurt, 150-gram cold heavy cream, one teaspoon of vanilla bean paste, If using a fresh vanilla bean, use a sharp knife to split 1/2 the vanilla bean lengthwise, scrape out the seeds, and add them to the mixing bowl.
   
• 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.
   
• Take the refrigerated cold passion fruit puree out of the fridge, add it to the whipped yogurt frosting, and combine lightly with a spatula.
   
• Take the cake out of the fridge, remove the plastic wrap, Pile the Passion Fruit Yogurt Frosting onto the middle of the top, and use the spatula to cover the cake in an old fashion way all over the cake.
   
• For maximum freshness and deliciousness, serve immediately or store leftovers in the fridge, covered, for up to 2 days.