Position rack in center of oven; preheat to 350°F. Butter two 9-inch-diameter pans with 1 3/4-inch-high sides. Dust with flour; discard excess. Bring 3/4 cup water to boil in medium saucepan. Remove from heat. Whisk in cocoa to form paste. Cool slightly. Add buttermilk; whisk until blended.
Whisk flour and next 3 ingredients in medium bowl to blend. Using electric mixer, beat sugar and next 4 ingredients in large bowl until light, about 5 minutes. Beat in cocoa mixture. Add dry ingredients and beat just until blended. Divide batter between prepared pans.
Bake cakes until tester inserted into center comes out clean, about 20 minutes. Cool cakes in pans on rack 10 minutes. Turn cakes out onto racks; cool.
Place 1 cake layer on platter. Spread 3/4 cup frosting over. Top with second layer. Spread remaining frosting over entire cake. (Can be made 1 day ahead. Cover with cake dome; let stand at room temperature.)