Step 1

Preheat the oven to 350 degrees Fahrenheit. Grease two (8-inch) round cake pans with butter. Line it with parchment paper, and then grease the lining with butter and dust with flour.

Step 2

Sift the flour, sugar, cocoa powder, baking soda, baking powder, and salt in a large bowl. Mix on low speed with an electric mixer with a paddle attachment just until well combined.

Step 3

In a separate bowl, whisk together the buttermilk, oil, eggs, and vanilla. Run the mixer on low speed and slowly pour the wet mixture into the dry. Beat in the coffee, scraping the bottom and sides of the bowl with a rubber spatula.

Step 4

Pour the batter into the prepared pans and bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean. Cool the cakes in the pans for 30 minutes, and then flip them over onto a wire rack to cool completely.

Step 5

Meanwhile, make the frosting. Chop the chocolate and place it in a large, heat-proof bowl. Set it in a pot over simmering water. Stir until smooth and melted. Set aside to cool.

Step 6

In a large bowl, beat butter on medium-high speed until light and fluffy, about 3 minutes. Add the egg yolk and vanilla, and continue beating for 3 minutes.

Step 7

Reduce the mixer speed to medium and gradually add the powdered sugar, scraping the bottom and sides of the bowl, until smooth and creamy.

Step 8

Dissolve the coffee powder in 2 teaspoons of hot water. While the mixer is running at low speed, pour the chocolate and coffee into the bowl of frosting. Mix just until blended.

Step 9

Assemble the cake. Place one layer of cake, flat side up, on a flat plate. Using an offset spatula, spread half of the frosting evenly on top. Place the 2nd layer of cake on top, rounded side up. Spread the remaining frosting on top and sides of the cake.

Step 10

Refrigerate the cake for at least 1 hour before serving. Slice and enjoy!