Beat butter at medium speed with an electric mixer about 2 minutes or until soft and creamy. Gradually add sugar, beating a medium speed 5 to 7 minutes. Add eggs, 1 at a time, beating just until yellow disappears.

Combine flour, baking powder, and salt; add to butter mixture alternately with milk, beginning and ending with flour mixture. Mix at low speed after each addition just until blended. Sir in vanilla. Remove 2 cups batter, and stir in cocoa. Spoon one-third of remaining batter into a greased and floured 13-cup Bundt pan; top with half of chocolate batter. Repeat layers, ending with plain batter. Gently swirl batter with a knife. Bake at 325 degrees Fahrenheit for 1 hour and 5 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on wire rack for 10 minutes; remove from pan, and let cool completely on a wire rack (outer edge may stick to pan). Sprinkle with powdered sugar. Yield: 1 (10-inch) cake.