Preheat oven to 350 degrees. Whisk together cocoa and water; cool. Set aside. Combine eggs, 1/4 of the cocoa mixture and vanilla. In separate bowl, combine dry ingredients; add butter and remaining cocoa mixture. Mix on low speed to moisten. Beat on medium speed 1-1/2 minutes. Gradually add egg mixture in thirds mixing well after each addition. Spread batter into 2 prepared 9-inch layer pans. Bake until tester inserted comes out clean, 25 to 30 minutes. Cool cakes in pan 10 minutes. Remove from pans; cool. Frost with favorite frosting.