Bakery Style Chocolate Sheet Cake

• Adjust the oven rack to the middle position and 30 minutes before baking, preheat your oven to 350F°/175C°.
   
• Grease and line a 40 x 30 cm ( 16 x 12 inch) sheet pan.
   
• When you are ready to mix your cake, warm three large eggs (in the shell) by placing them in a bowl of hot tap water for 5-10 minutes ( You may need to change the water once to get them warmed up) before you start making the cake, this step will help your eggs whip up better and take on more air, and the sugar will dissolve better.
   
• In a medium bowl, sift the 330-gram all-purpose flour, 1 1/2 teaspoon baking powder, and 1 1/2 teaspoon baking soda, add 3/4 teaspoon fine sea salt, combine and set aside for now.
   
• Place 100-gram unsweetened cocoa powder and two teaspoons of instant espresso powder (optional) in a medium heatproof bowl. Whisk in 200 gram freshly brewed warm coffee (preferably espresso) until fully combined and smooth. Coffee enhanced this chocolate cake and brings out its intense chocolate flavor, making the cake yummier. Next, stir in the 250-gram plain yogurt. The yogurt will lower the temperature of the cocoa mixture, so it is good to go right away. Set aside for now.
   
• Into the bowl of a stand mixer fitted with the whisk attachment, put 355 gram superfine, 150-gram eggs (from about 3 large eggs, use a scale), and 1/2 tablespoon vanilla bean paste or pure vanilla extract.
   
• Whip the egg mixture on medium speed ( never on high) until triple in volume, very pale yellow in color, and mousselike and thick in texture—about 8 to 10 minutes.
   
• Lift the beater to check the texture. The batter should fall in a slow, thick ribbon and take a couple of seconds to disappear. It is important to whip until the texture is achieved. I prefer to set a timer for at least 8 minutes. 
   
• Add one-third of the flour mixture and one-third of the cocoa-coffee-yogurt mixture and mix on low speed until just almost combined about 10 to 15 seconds. Stop the mixer and scrape the sides and bottom of the bowl, and fold the batter to ensure everything has been incorporated.
   
• Repeat twice, alternating between the dry and wet ingredients, until both are incorporated, about 10 to 15 seconds each time, not any longer, scrape the sides and bottom of the bowl and fold the batter to ensure everything has been incorporated each time. Do not overmix.
   
• With the mixer on low speed, in a slow and steady stream pour the slightly cooled melted butter into the mixer bowl. This step takes about 20 seconds, Do not overmix!
   
• Remove the bowl from the mixer, scrape the sides and bottom of the bowl, and fold the batter one more time.
   
• Pour the cake batter into the prepared sheet pan and use an offset spatula or silicone spatula to spread it evenly.
   
• Bake the cake in the center of the oven until a toothpick inserted into the center comes out clean, 23 to 25 minutes. Mine was done at 25 minutes.
   
• Let the cake cool completely in the pan for about 1 hour.
   
• You can leave the cake in the sheet pan and cover the cake later on with swiss meringue chocolate buttercream, or turn the cake onto a cake board as I do and remove the parchment paper.
   
• The chocolate sheet cake without the frosting will keep fresh for 2 to 3 days ( make sure to wrap it in plastic wrap) and it freezes well if you would like to bake it ahead of time.

Swiss Meringue Chocolate Buttercream
 

• For this frosting, try using a good quality bittersweet chocolate for the best flavor, I use Valrhona 66%, but Lindt excellence bars ( 70%) or Callebaut ( 70%) is a great brand to use as well. Do not use chocolate chips, which are designed "Not" to melt in chocolate cookies and won't taste good in this Swiss Meringue Chocolate Buttercream. 
   
• For the buttercream, it is important that the butter is semi-cold when you whip it whipped fluffy on high speed. Also, the whipped butter should not feel greasy if it feels greasy place the bowl with whipped butter back in the fridge, then re-whip for 10 seconds on high speed just before adding it to the whipped meringue.
   
• For the buttercream, In the bowl of an electric mixer, fitted with the whisk attachment, beat 175-gram semi-cold salted butter and 175-gram semi-cold unsalted butter on high speed for about 5 minutes, or until light and fluffy, thick and white pale in color. Make sure to scrape down the sides of the bowl a few times. Set aside for now.
   
•  Melt 150-gram bittersweet chocolate 66 to 70% ( coarsely chopped or callets) in a medium heatproof bowl over a water bath, Chocolate should melt slowly. Remove it from the hot water before it is completely melted and then stir until it is entirely melted and smooth. Or melt the chocolate in the microwave on 50 percent powder in four 15-seconds increments, stirring after each, for a total of 60 seconds, Stir until almost completely melted. Set aside to cool down slightly. The melted chocolate needs to cool down just slightly so keep an eye on it so that it doesn't harden.
   
• For the meringue, Pour about 4 cm ( 2- inches ) of water into a medium saucepan and place over medium heat, bring to a boil, then lower the heat to maintain a gentle simmer.
   
• For best results, use fresh egg whites (often boxed whites don’t whip up).
   
• Cleanly separate the egg whites from the yolks. 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.
   
• Wipe your mixer bowl, your whisk of an electric mixer, and your hand whisks with a paper towel dampened with a bit of lemon juice to eliminate any trace of grease. 
   
•  Place 275 grams superfine sugar, 165 grams egg whites and 1/4 teaspoon cream of tartar into the bowl of a stand mixer. Whisk the ingredient together with a clean whisk to make a thick slurry.
   
• Place the mixer bowl over a gentle simmer. Do not let the base of the bowl touch the hot water below. 
   
•  Whisking constantly until the sugar is 100% dissolved and the mixture has the color of milk or until it reaches a temperature between 60C°/140F° if using a digital thermometer or candy thermometer. This step takes 5 to 7 minutes.
   
• Be sure to wipe down the sides of the bowl as you warm the mixture as often the sugar clings to the side of the bowl and makes your meringue grainy.
   
• As soon as the sugar is 100 % dissolved,  Set the bowl onto the stand mixer fitted with the whisk attachment. 
   
• Whip on medium speed for about 5 to 7 minutes or until the meringue is fluffy, light, and with stand-up peaks, and more importantly until the mixing bowl is no longer warm to the touch. About 5 minutes. Stop the mixer and scrape the side and the bottom of the bowl.
   
• Change the whisk to the paddle attachment, with the mixer at low-speed beat in the whipped butter, a tablespoon at a time, ( this helps keep the temperature stable) until it is all used up. This step takes about 2-3 minutes. 
   
• Stop the mixer, scrape the side and the bottom of the bowl, add the 150 gram slightly cooled melted chocolate and beat on low speed for 2 to 3 minutes or until fully incorporated, and is fluffy, and thick. The chocolate buttercream lightens in color quite a bit depending on what mixer speed you are using after you have added the melted chocolate. I prefer mixing on low speed it makes for a chocolate frosting with fewer air pockets that is silky smooth.
   
•  If you plan to add instant espresso coffee and unsweetened cocoa powder sift 2 teaspoons of instant espresso powder and sift 2 to 3 tablespoons of unsweetened cocoa powder in the mixing bowl add the same time when you add the melted chocolate. The unsweetened cocoa powder gives the frosting a more intense chocolate flavor. 
   
• If the Swiss Meringue Chocolate Buttercream is not firm enough refrigerate the bowl for 10 to 20 minutes, lightly re-whip on low speed if needed, or grab a spatula and stir the frosting for a few minutes.

• Scoop the whipped chocolate buttercream onto the cooled chocolate sheet cake and spread it evenly, using an offset spatula to create peaks or swirls on top of the chocolate sheet cake.
   
• Serve and enjoy it is so delicious.
   
• This cake keeps tightly covered at room temperature or in the fridge for up to 3 days