18tablespoonsunsalted butterroom temperature and cut in cubes
1 ¾cupsunsweetened baking cocoa
3 ½cupsconfectioners' sugar
1teaspoonespresso powderoptional, for enhanced chocolate flavor
1cupunsalted buttervery soft
4 - 6tablespoonsheavy cream
Grease and flour 3 6-inch baking pans and add parchment to the bottoms. Preheat the oven to 350 °F.
Create a double boiler by placing a small amount of water in a small saucepan and heat until boiling. Place a large heatproof bowl over the saucepan and turn of the heat. Add the chocolate, hot water, cocoa powder, and ½ cup of sugar. Stir well until the chocolate is melted and the mixture is smooth.
In a small bowl, mix together the flour, baking soda, and salt. Set aside.
Place the buttermilk and vanilla in another small bowl and set aside.
In a mixer add the eggs, yolk, and remaining sugar. Whip on high for about 5 minutes.
Add the chocolate mixture and beat until well-combined.
Add the butter one cube at a time, allowing the butter to incorporate before adding another cube of butter.
Alternate between adding the flour and buttermilk mixture. Start and end with the flour which should be added in three additions, while the buttermilk mixture is added in two additions.
Evenly divide the batter among the cake pans and bake for about 18 - 22 minutes, or until a toothpick inserted in the middles comes back with a few crumbs sticking to it.
Allow the cakes to cool in the pans for about 15 mins, then turn them out onto wire racks to cool completely. Tightly wrap each layer as you prepare the frosting.
Make the frosting. Mix together the cocoa powder, powdered sugar, and espresso powder together.
Beat the cocoa mixture, butter, and salt together. The mixture may seem lumpy, and that is normal.
Add in the heavy cream one tablespoon at a time and continue to beat on medium speed until it a smooth, creamy consistency.
Remove the cake layers from the freezer and level them as needed. Add frosting to the layers and frost the outside as normal.
I made this cake in 3 6-inch cake pans, but you can make it in 2 9-inch cake pans like the original recipe from America's Test Kitchen, simply increase the baking time to 25 - 30 mins.
The frosting recipe is not from America's Test Kitchen.