Prepare the Cake

Preheat the oven.

• Preheat the oven to 350°F (175°C).

Prepare the cake pans.

• Grease three 8-inch round cake pans and line the bottoms with parchment paper. Set aside.

Combine the dry ingredients.

• In a medium mixing bowl, whisk together the all-purpose flour, cocoa powder, cornstarch, baking soda, baking powder, and salt until fully combined.

Combine the dairy mixture.

• In a small bowl or measuring glass, stir together the buttermilk, sour cream, and vanilla extract until smooth.

Combine the sugars and oil.

• In a separate large mixing bowl, whisk the granulated sugar, light brown sugar, and vegetable oil by hand until smooth.

Add the eggs and egg yolks.

• Add the eggs and egg yolks one at a time, whisking well after each addition until fully incorporated.

Add the buttermilk mixture.

• Add the buttermilk mixture in two additions, whisking well after each addition.

Add the dry ingredients and coffee.

• Alternate adding the dry ingredient mixture and the hot coffee to the batter, beginning and ending with the dry ingredients. Whisk gently after each addition. The batter will be thin.

Divide the batter.

• Divide the batter evenly among the prepared cake pans.

Bake the cake layers.

• Bake for 30 to 35 minutes, or until a toothpick inserted into the center of each cake layer comes out clean.

Cool the cake layers.

• Allow the cakes to cool in the pans for 10 to 15 minutes. Turn them out onto a wire rack and allow them to cool completely before frosting.

Prepare the Chocolate Fudge Frosting

Prepare a double boiler.

• Place 2 to 3 inches of water in a medium saucepan. Set a large heatproof bowl over the saucepan, ensuring the bottom of the bowl does not touch the water.

Melt the chocolate and cream.

• Add the chocolate and heavy whipping cream to the heatproof bowl. Heat over medium heat, stirring as the water simmers underneath, until the mixture is smooth.

Remove from heat.

• Carefully remove the bowl from the heat. Use an oven glove or towel to protect your hands from steam.

Add the remaining frosting ingredients.

• Add the butter, salt, and powdered sugar to the melted chocolate mixture.

Mix the frosting.

• Stir gently until the powdered sugar is no longer visible, then mix more thoroughly until the frosting is smooth and glossy.

Cool the frosting.

• Cover the frosting and allow it to cool until thickened and spreadable.

Assemble the Cake

Add the first cake layer.

• Place one cooled cake layer on a serving plate or cake stand.

Add frosting.

• Spread one-fourth of the thickened frosting evenly over the first cake layer.

Add the second cake layer.

• Place the second cake layer on top and spread another one-fourth of the frosting evenly over it.

Add the final cake layer.

• Place the final cake layer on top.

Frost the cake.

• Use the remaining frosting to coat the top and sides of the cake evenly. Pipe any excess frosting on top, if desired.

Chill before serving.

• Refrigerate the cake for 30 to 60 minutes to allow the layers and frosting to set.

Slice and serve.

• Slice the cake and serve.