Preheat oven to 300 degrees F and grease and flour a 12-cup bundt pan.
Beat butter and sugar together with electric beaters until light and fluffy. Add eggs, one at a time, mixing well after each addition. Beat in sour cream and vanilla. Sift flour, baking powder, baking soda and salt and add to cake in three additions, mixing gently after each addition.
Remove 2 cups of batter and stir in melted bittersweet chocolate. Stir dark chocolate chips into white cake batter and stir white chocolate chips into dark batter. Dollop the different batters into prepared pan alternately, and run a knife through the cake once, swirling the batter just a little. Bake cake for 50 to 60 minutes, until a tester inserted in the center of the cake comes out clean. Allow cake to cool for 20 minutes before turning out to cool completely.