- 1
Preheat oven to 350°F. Butter and flour three 9-inch cake pans with 1½-inch high sides; set aside.
- 2
Place the chopped chocolate and butter in a large metal bowl. Set the bowl over a saucepan of barely simmering water and stir until chocolate is melted and smooth. Remove bowl from over water. Stir in Guinness and set aside.
- 3
In a medium bowl, beat sugar, eggs and yolks until mixture is light and fluffy, about 3 minutes. Beat in oil and sour cream until well combined. Slowly add the chocolate mixture, mixing until ingredients are well incorporated.
- 4
In a separate bowl, whisk together flour, baking powder, 1 Tbsp. espresso powder, cocoa powder and salt. Beat flour mixture into chocolate mixture in two additions; mix until just incorporated, being careful not to overmix.
- 5
Pour batter evenly into the three prepared cake pans. Bake for 30 to 40 minutes, or until a toothpick inserted into the center comes out clean.
- 6
Allow to cool in pans for 20 minutes. Remove cake from pans and allow to finish cooling. Once cool, transfer to the fridge and chill for at least 1 hour prior to assembling.
- 7
To prepare the frosting: place chopped bittersweet chocolate in a medium heatproof bowl. Combine whipping cream and remaining 1 tsp. espresso powder in a small saucepan. Bring mixture to a simmer over medium-high heat, whisking occasionally. Pour cream mixture over chocolate and whisk until smooth. Chill frosting until slightly thickened, about 2 hours.
- 8
To assemble the cake, use a serrated knife to trim the rounded tops from the cake layers so that the tops are flat. Place one cake layer, trimmed side down, on your desired serving platter. Drop 1 cup frosting over the cake layer; spread frosting to edges with an offset spatula or butter knife. Top with second cake layer, trimmed side up. Repeat with 1 cup of frosting. Top with final cake layer, trimmed side down. Spread remaining frosting evenly over top and sides of cake.
