1. Heat oven to 350°F. Grease three 9-inch round baking pans. Line bottoms with wax paper; grease and flour paper. Mix cocoa with boiling water in small bowl; stir until smooth. Set aside to cool.
2. Beat butter and sugar in large bowl until fluffy. Add eggs, one at a time, beating well after each addition. Stir in vanilla. Gradually add cocoa mixture, beating well.
3. Stir together flour, baking soda and salt; add alternately with milk and sour cream to butter mixture, beating until blended. Pour batter into prepared pans.
4. Bake 30 to 35 minutes or until wooden pick inserted in center comes out clean. Cool 10 minutes. Remove from pans to wire racks; carefully peel off wax paper. Cool completely. Frost as desired. 10 to 12 servings.