(makes two 8-inch pans, or, four Brooklyn Blackout layers)
6 1/2 tablespoons unsalted butter, at room temperature1 1/4 cups granulated sugar2 eggs, at room temperature1/4 teaspoon pure vanilla extract1/3 cup unsweetened cocoa powder3/4 teaspoon baking powder3/4 teaspoon baking sodaa pinch of salt1 1/3 cups all-purpose flour2/3 cup whole milk
For the Chocolate Custard Frosting:
(makes enough for one 8-inch, 4-layer cake)
2 1/2 cups water plus 1/2 cup water, separated2 1/2 cups granulated sugar1 tablespoon light corn syrup1 cup unsweetened cocoa powdera scant 2/3 cup cornstarch5 1/2 tablespoons unsalted butter, cold and cut into 1-inch cubes1/2 teaspoon pure vanilla extract
For the Chocolate Cake Base:
Preheat the oven to 325. Prepare two 8-inch round baking pans by lining the bottom with parchment paper, and spraying both the parchment paper and the pan liberally with cooking spray
In a freestanding electric mixer with a paddle attachment (or, use a handheld electric whisk), combine 6 1/2 tablespoons unsalted butter and 1 1/4 cups granulated sugar and cream until light and fluffy, about 5 minutes. Add 2 eggs one at a time, only adding the next egg when the first egg has been fully incorporated into the mixture.
Scrape down the sides of the bowl with a rubber spatula. Turn the mixer back on to its lowest speed and beat in 1/4 teaspoon pure vanilla extract, 1/3 cup unsweetened cocoa powder, 3/4 teaspoon baking powder, 3/4 teaspoon baking soda and a pinch of salt. Continue mixing until just combined, then add half of of the 1 1/3 cup flour, then 2/3 cup cup whole milk, and finish with the remainder of the flour. Continue mixing until just combined.
Divide the batter evenly into 2 pans, using a rubber spatula to spread the batter evenly across its pan. Bake in the preheated oven for 25 to 30 minutes, or until a skewer inserted into the center of the cake comes out clean and the tops of the cakes bounce back when gently poked with your finger. Let the cake layers cool slightly in the pans before turning out onto a wire rack to cool completely. Remove the parchment circles from the tops of the cakes.
For the Chocolate Custard: Combine 2 1/2 cups water, 2 1/2 cups granulated sugar, 1 tablespoon light corn syrup, and 1 cup unsweetened cocoa powder into a large sauce pot and bring to a boil over medium heat, whisking occasionally.
In the bowl of a freestanding electric mixer fitted with a whisk attachment (or, use a handheld whisk), combine 2/3 cup cornstarch with the remaining 1/2 cup water, whisking on medium speed as you add the water to the cornstarch. The mixture should be the consistency of thick glue, and should be relatively lump-free. Keep mixing until all the major lumps have dissolved.
Once the cornstarch mixture is the appropriate thickness, transfer to a liquid measuring cup and slowly pour into the cocoa mixture in the pan (from the first step), whisking constantly. Keep at medium heat, but bring back to a boil, whisking constantly. Continue cooking until the mixture is quite thick — this should take at least 5 to 10 minutes.
Once the mixture has thickened, remove from heat and use a rubber spatula to stir in 5 1/2 tablespoons unsalted butter and 1/2 teaspoon pure vanilla extract until completely melted and fully incorporated into the mixture.
At this point, the custard should be thick and shiny, but still pliable and easy to work with. It should have the texture of really soft butter. Transfer the custard to a 9 x 13 inch baking pan or glass casserole dish, spreading the custard out evenly across the pan. Cover with plastic wrap, pressing the plastic wrap directly on the custard to prevent a skin from forming. Chill until completely cool, preferably overnight. When the custard is ready to use, it should have the texture of margarine.
Putting It All Together:
Use a serrated knife to level the 2 chocolate cakes by slicing off the domes off the top of each cake. Save the excess cake (also known as the baker's reward) and set aside. Using your serrated knife, cut each cake into two individual pieces by carefully slicing each cake horizontally through its center. At this point, you should have 4 thin cakes for each layer of the cake.
Put one layer on a cake stand and spread 1/4 cup of the chocolate custard over it. Place the second layer on top and spread another 1/4 cup of custard over it. Repeat until all layers are used, spreading the remaining custard over the top and sides of the cake.
Take the excess cake (baker's reward) that you saved from the first step and place them in a food processor, processing into crumbs. Cover the freshly-frosted cake with the crumbs and chill for 2 hours, before bringing back to room temperature and serving.