Cocoa Layer Cake

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