Heat oven to 350 degrees F. Grease and flour pan; set aside. In large bowl, combine cake mix, eggs, sour cream, oil, water, sugar and vanilla. Beat on medium speed, 2 minutes, scraping bowl often. Remove one-third of the batter and place in small bowl; set remaining batter aside. Stir melted chocolate into batter in small bowl. Spoon half of the vanilla batter and half of the chocolate batter into prepared pan. Using a small spatula, gently swirl batters together. Spoon
remaining batters into pan and swirl again. Bake for 40-45 minutes, until toothpick inserted comes out clean. Cool 10 minutes in pan; invert onto cooling rack and cool cake completely. Decorate as desired.