In a large bowl, with electric mixer at low speed, beat sugar and butter or margarine until blended.Increase speed to high, and beat until light and fluffy.Add flour, milk, baking powder, vanilla, salt, and eggs; beat at medium speed until well mixed.Increase speed to high, and beat batter 4 minutes longer.
Remove about 2 1/2 cups batter to a medium bowl.With a wire whisk or fork, beat cocoa into batter in medium bowl until well blended.
Grease a 10 inch tube pan. Alternately spoon vanilla and chocolate layers into prepared pan.With a large spoon, cut and twist through batters to obtain marbled effect.
Bake at 350 degrees F (175 degrees C) for 1 hour, or until a toothpick inserted in the center of the cake comes out clean.Cool cake in pan, and on a wire rack, for 10 minutes.Remove cake from pan, and cool completely.
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