Step 1

Preheat the oven to 350°F and grease to 8-inch round cake pans with butter. Line bottom with parchment paper, followed by more butter, then flour the pans, shaking out excess.

Step 2

Sift together the flour, sugar, cocoa powder, baking soda, baking powder, and salt directly into the bowl of a stand mixer. Using the paddle attachment, mix on low speed until combined.

Step 3

In a separate bowl, mix together the buttermilk, oil, eggs, and vanilla. With the mixer speed on low, gradually add the wet ingredients to the dry. Add coffee and mix just to combine, scraping the bowl with a rubber spatula as needed.

Step 4

Pour batter into prepared pans and bake until a toothpick inserted into the center comes out clean, 35-40 minutes. Let the cakes cool in the pans for 30 minutes, then invert them onto a cooling rack to cool completely.

Step 5

When cakes are nearly cool, make the buttercream:

Step 6

Chop the chocolate and place it in a bowl over a pan of lightly simmering water (or double boiler). Stir until just melted, then set aside to cool.

Step 7

In the bowl of a stand mixer, beat the butter with the paddle attachment on medium-high speed for 3 minutes until light yellow and fluffy. Add vanilla and egg yolk and beat 3 minutes more.

Step 8

Turn mixer to low, then gradually add powdered sugar. Once combined, increase speed to medium and beat until smooth and creamy, scarping down sides and bottom of bowl as needed.

Step 9

Dissolve the coffee in 2 teaspoons of very hot tap water. With mixer on low, gradually add the coffee and chocolate. Mix until blended and frost the cooled cake immediately. Enjoy!