Heat oven to 350°F Grease a 10" tube pan.
Beat sugar and butter until blended; then increase speed to high and beat until light and fluffy.
Add flour and remaining ingredients EXCEPT cocoa. Beat at low speed until well mixed, scraping bowl often; then increase to high and beat 4 minutes longer, scraping bowl as needed.
Scoop out 2 1/2 cups of batter into a small bowl. Whisk in cocoa powder.
Alternately spoon vanilla and chocolate batters into prepared tube pan. Using a knife, cut and twist through batter to create a marble effect.
Bake 60 minutes or until a toothpick comes out clean.
Cool in pan on a wire rack for 10 minutes. Then run a spatula or knife around the pan to loosen the cake. Remove cake and cook on a wire rack.