This chocolate sheet cake might have a few steps to follow, but it's easy to make. I've included detailed baker's notes with the recipe, not to overwhelm you, but to help ensure your cake is as moist and tender as possible. These extra bits of info are just my way of guiding you to a great result. So, don't worry about the length of the text—those tips are there to guarantee your baking success. Happy baking!

Moist & Tender Chocolate Sheet Cake

• Baker Note; “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.”
   
•  Grease and line a 40 x 30 cm ( 16 x 12 inch) sheet pan.
   
•  Adjust the oven rack to the center of the oven and preheat the oven to 175C°/350F° about 30 minutes before baking. 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. 
   
• Bakers Note; Using superfine sugar is a great trick for achieving the finest texture in this chocolate sheet cake. It's easily made at home by blending regular white sugar. Simply process it in your blender for 6 seconds, give the container a shake to redistribute the sugar, and then blend for another 6 seconds. This method produces sugar with a finer grain that dissolves more readily, giving you a smoother mixture. If you prefer not to take this extra step, regular white sugar will work as well, though superfine sugar does have its advantages in terms of dissolving more easily.
   
• Place 5 large eggs ( in their shells) in a bowl of tepid, not hot, water for 5 minutes while you gather the other ingredients. This will help your eggs and sugar whip up better, ensuring that all the sugar dissolves, resulting in a better rise for your chocolate sheet cake. 
   
• In a measuring cup, combine 275 grams of plain full-fat yogurt with 275 grams of full-fat sour cream. Whisk them together briefly to ensure they're well mixed. The mixture should be at room temperature, around 23°C (73.4°F), for optimal incorporation into your recipe. If you forget to take the sour cream and yogurt out of the fridge to reach room temperature, a quick solution is to place the bowl in lukewarm water for about 5 minutes. This gentle method will bring the ingredients to the desired temperature without overheating them.
   
• In a large bowl, sift together 435 grams of all-purpose flour or pastry flour, 1 1/2 teaspoons baking powder, 1 1/2 teaspoons baking soda, and 75 grams of unsweetened cocoa powder, To get the chocolate sheet cake you truly deserve, sift the flour /cocoa mixture twice to get the flour and cocoa powder as aerated and light as possible. Set aside for now.
   
• In a small saucepan melt 250 grams salted or unsalted butter over medium heat,  Remove the saucepan from heat and stir in 50 grams of dark chocolate, and 1/4 teaspoon Maldon sea salt or kosher salt with your spatula or a small whisk slowly stir until the butter and chocolate are combined. Transfer to a small bowl or measuring cup and let stand until tepid, about 37°C/99F°. 
   
• In a small coffee cup combine 150 gram of brewed coffee and 1 tablespoon of espresso powder, and let stand until tepid. about 37°C/99F°.
   
• In the bowl of a stand mixer equipped with the whisk attachment, combine 465 grams of superfine sugar, 230 grams of eggs, and 2 teaspoons of vanilla bean paste. Begin whipping the mixture on low speed for 3 minutes—this slower initial mixing helps the sugar to dissolve more effectively. Then, increase the speed to high and continue to whip for 8 minutes, before switching to medium speed for a final 3 minutes. By the end of this process, you should have a mixture that is pale and has thickened noticeably. To check if it's ready, stop the mixer and lift the whisk; the batter should flow off in a ribbon that allows you to trace a figure eight ('8') that holds its shape momentarily before sinking back into the mix. This 'figure eight test' is a classic method to ensure that your mixture has reached the proper consistency for a light, airy cake.
   
• Set your mixer to its lowest speed and slowly pour in the tepid butter and chocolate mixture, This step takes about one minute.
   
• Add the dry ingredients in three rounds, alternating with the yogurt/sour cream and the lukewarm coffee, and mix on low speed with each addition for no more than five seconds. Don't be concerned if the batter looks slightly uneven or you can still see spots of flour or cocoa powder after the final addition—that's expected and preferable to overmixing, which can lead to a less tender cake. Once all the ingredients are added, remove the bowl from the mixer. Take a balloon whisk and fold the ingredients together gently, just enough to combine them. The goal is to mix as little as necessary; a gentle hand here is essential for achieving a cake that's fluffy and tender.
   
• Pour the chocolate cake batter into a sheet pan, once the batter is in the pan, use a spatula to spread it evenly. Before baking, gently spin the pan 2 times; this helps the batter set against the sides, aiding in a better rise. Importantly, do not tap the pan on the kitchen counter as this can eliminate the air bubbles you've just incorporated, deflating the batter and reducing the cake's volume.
   
• Bake immediately in the center of the oven for about 30 to 35 minutes. The best way to determine if your chocolate sheet pan cake is done, keep an eye on the center. You want it to be nicely risen. Try not to open the oven to check until at least 30 minutes have passed. If through the oven window the center doesn't seem quite risen, give it a few more minutes, then test with a cake tester or skewer if it comes out clean, the cake is ready. If not, it may need a few more minutes of baking. Mine was done at 30 minutes.
   
• Remove the chocolate sheet cake from the oven, While the cake is cooling for a few minutes, prepare the chocolate icing. 
   
• Bakers Note; For extra delicious you can also poke holes in the warm cake just before you pour the warm chocolate icing on top.
   
• For the chocolate icing, melt 125 grams salted or unsalted in a medium saucepan over medium heat. Meanwhile combine 75 grams brewed coffee and 2 teaspoons instant espresso powder in your coffee mug, In a separate bowl, whisk together 375 grams of powdered sugar with 3 tablespoons of unsweetened cocoa powder, 1/4 teaspoon of Maldon sea salt or kosher salt, and 2 teaspoons of vanilla bean paste until combined. Then add 75 grams of warm coffee to the dry ingredients, followed by the melted butter. Stir this mixture until it's smooth and free of lumps.
   
• Once your chocolate icing is ready, pour it over the warm cake, aiming for an even distribution across the top. The cake's residual warmth will help the icing spread. If necessary, a spatula can be used to smooth it out. This straightforward approach ensures your chocolate sheet cake is topped with a rich, smooth layer of icing that complements the moist cake beneath.
   
• Enjoy a piece of this chocolate sheet cake while still warm or wait until the icing gets set.
   
• Cutting the chocolate sheet cake into pieces will serve about 24 to 30 pieces. These are approximate numbers and can be adjusted based on how you prefer to portion the cake.
   
• This chocolate cake will stay delicious for about 5 days if you keep it wrapped up tight. Just use some plastic wrap to cover it at room temperature, or pop it in the fridge if you prefer. Chilling turns the cake a bit fudgier, which is great if that's your thing. But if you love your cake super soft and tender, keep it out on the counter—covered, of course. Either way, you're in for a treat that lasts all week!