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For best results, bake your cake in a 12-cup fluted tube cake pan. Prepare cake mix according to package instructions.
To add the chocolate swirl:
Remove 1 cup of the batter; stir in 1/4 cup cocoa powder mixed with 1 tablespoon water. Pour remaining batter into pan. Drop chocolate batter by about 12 tablespoonfuls onto batter in pan. Cut through batters with knife for marbled design.
Bake 50 to 55 minutes or until toothpick inserted in center comes out clean. Cool 15 minutes in pan on wire rack; remove from pan to wire rack. Cool at least 1 hour 30 minutes. Serve warm or cool.
Answered 01/18/2013 by mgoetz