Step 1

Preheat the oven to 350°F (175°C). Grease three 8-inch round cake pans and line them with parchment paper. Set aside.

Step 2

Make the Cake: In a medium bowl, whisk the flour, cocoa powder, cornstarch, baking soda, baking powder, and salt until well combined. Set aside.

Step 3

Stir the buttermilk, sour cream, and vanilla in a small bowl or measuring glass until smooth. Set aside.

Step 4

In a separate large bowl, whisk the sugars with the oil by hand until smooth. Add the eggs and yolks one at a time and whisk to combine. Then, add the buttermilk mixture in two parts, whisking well after each addition.

Step 5

Alternate adding the dry ingredients and hot coffee, starting and ending with the flour. Whisk gently between each addition. The batter will be thin.

Step 6

Divide the batter evenly between the prepared pans and bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.

Step 7

Let the cakes cool in pans for 10-15 minutes, then turn them onto a wire rack to cool completely.

Step 8

Make the Fudge Frosting: Combine the chocolate and cream in a large, heatproof bowl. Place the bowl on the rim of a medium saucepan with 2-3 inches of water in the bottom (the bottom of the bowl should not touch the water). Turn the heat to medium and stir the chocolate and cream as the water simmers underneath. Use an oven glove to hold the bowl and protect your skin from the steam.

Step 9

When the ganache is smooth, remove the bowl from the heat and add the butter, salt, and powdered sugar. Mix gently until the sugar is no longer powdery, then mix more vigorously until smooth and shiny. Cover and set aside to cool and thicken.

Step 10

Assemble: Place one cake layer on a serving plate. Add 1/4 of the thick frosting and spread evenly. Repeat with the next layer of cake and 1/4 of the frosting, then add the last cake layer on top. Cover the cake with the remaining frosting, evenly coating the top and sides. Use any excess to pipe on top, if desired.

Step 11

Chill the cake for 30-60 minutes for the layers to set, then slice, serve, and enjoy!