Preheat oven to 350 degrees F.
For the cake, prepare 3 baking pans by buttering the pans. Whisk cocoa and 1/2 cup lukewarm water in a small bowl. Whisk the remaining 1/2 cup of the lukewarm water with the buttermilk in another small bowl. Sift flour, baking soda and salt into a 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, and then beat until smooth and fluffy, about 15 seconds. Add cocoa mixture; beat to blend. Add flour mixture in 3 additions alternately with buttermilk in 3 additions, beating to blend after each addition. Divide batter among the 2 cookie sheets (about 2 ½ cups batter per pan)