Step 1

Preheat oven to 350°F. Lightly coat three 8-inch round cake pans with 2-inch-high sides with nonstick spray or butter and line with parchment paper rounds, then spray paper. Set aside.

Step 2

In heavy large saucepan over medium heat, bring Guinness, coffee and butter to a simmer. Add cocoa powder and whisk until mixture is smooth. Cool slightly.

Step 3

Whisk flour, sugar, baking powder, and salt in a large bowl to blend. In another large bowl, use an electric mixer to beat eggs, vanilla and sour cream until smooth.

Step 4

Add Guinness-chocolate mixture to the egg mixture and beat until just combined, taking care the stout mixture is cool enough that it will not cook the eggs. Gradually add flour mixture, beating on low speed until just combined. Divide batter equally among prepared pans.

Step 5

Bake cakes 40-45 minutes, rotating pans between racks halfway through baking. Cakes are done when a tester inserted into center of cakes comes out clean. Transfer cakes to rack; cool 10 minutes in the pans. Turn cakes out onto rack and cool completely.

Step 6

Bring cream just to a simmer in a saucepan over medium heat. Remove from heat. Add chopped chocolate to a bowl and pour over hot cream. Let stand for a few minutes, then whisk until melted and smooth. Refrigerate until icing is spreadable, stirring every 20-30 minutes, about 1 ½ hours.

Step 7

Place 1 cake layer on plate. Spread 2/3 cup icing over. Top with second cake layer. Spread 2/3 cup icing over. Top with third cake layer. Spread remaining icing over top and sides of cake. Top with chocolate curls.