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Heat oven to 350°F Grease a 10" tube pan.
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Beat sugar and butter until blended; then increase speed to high and beat until light and fluffy.
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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.
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Scoop out 2 1/2 cups of batter into a small bowl. Whisk in cocoa powder.
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Alternately spoon vanilla and chocolate batters into prepared tube pan. Using a knife, cut and twist through batter to create a marble effect.
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Bake 60 minutes or until a toothpick comes out clean.
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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.