1Heat oven to 325°F. Grease and flour 12-cup fluted tube cake pan, or spray with baking spray with flour. In large bowl, beat cake mix, pudding mix, water, oil, vanilla and eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally.
2In medium bowl, mix powdered sugar and cocoa. Remove 1 1/4 cups batter; stir into cocoa mixture. Pour remaining batter into pan. Drop chocolate batter by generous tablespoonfuls randomly onto batter in pan. Cut through batter with knife for marbled effect.
3Bake 40 to 46 minutes or until toothpick inserted in center comes out clean. Cool in pan 15 minutes; turn upside down onto heatproof serving plate. Remove pan. Cool completely, about 1 hour.
4In microwavable bowl, microwave frosting uncovered on Medium 15 seconds. Spread over top of cake, allowing some to drizzle down side. Store loosely covered.