Cocoa Layer Cake

Preparation

1. For cake:

   Step 1

   Position 1 rack in top third and 1 rack in bottom third of oven; preheat to 350°F. Butter three 9-inch-diameter cake pans with 1 1/2-inch-high sides. Line with parchment paper rounds; butter parchment. Whisk cocoa and 1/2 cup warm water in small bowl. Whisk buttermilk and 1/2 cup water in another small bowl. Sift flour, baking soda, and 1/4 teaspoon salt into medium bowl. Using electric mixer, beat both sugars and butter in large bowl until pale yellow and fluffy (mixture will appear granular), about 5 minutes. With mixer running, gradually add beaten eggs, then beat until smooth and fluffy, about 15 seconds. Add cocoa mixture; beat to blend. Add flour mixture in 3 additions alternately with buttermilk mixture in 2 additions, beating to blend after each addition. Divide batter among pans (about 1 3/4 cups each).

   Step 2

   Bake cakes until tester inserted into center comes out clean, reversing pans halfway through baking, about 18 minutes. Cool completely in pans on racks.

2. For frosting:

   Step 3

   Melt butter in medium saucepan over medium heat. Stir in sugar, cocoa, espresso powder, and 1/2 teaspoon (scant) salt. Gradually stir in cream. Stir until mixture is very hot and just begins to simmer at edges. Reduce heat to low; stir 1 minute to let flavors blend. Transfer to medium bowl; stir in vanilla (frosting will resemble chocolate sauce). Chill until just thickened, stirring occasionally, about 1 1/2 hours. Let stand at room temperature.

   Step 4

   Run knife around cake sides. Carefully invert 1 cake onto plate (cake is very tender); peel off parchment. Spread with 1/2 cup frosting. Invert second cake onto palm of hand. Position cake 2 inches above frosted cake layer. Carefully slide cake onto first cake layer. Peel off parchment. Spread cake with 1⁄2 cup frosting. Repeat with third cake layer. Spread remaining frosting over top and sides of cake. DO AHEAD: Can be made 1 day ahead. Cover with cake dome; let stand at room temperature.

   Step 5

   Cut into wedges and serve.