PREHEAT oven to 375° F. Grease 10-cup bundt or tube pan.
MICROWAVE morsels in medium, uncovered, microwave-safe bowl on HIGH (100%) power for 1 minute; STIR. Morsels may retain some of their original shape. If necessary, microwave at additional 10 to 15-second intervals, stirring just until morsels are melted.
COMBINE cake mix, eggs, sour cream, vegetable oil, water and granulated sugar in large mixer bowl. Beat on low speed until moistened. Beat on high speed for 2 minutes.
STIR 2 cups batter into melted chocolate. Alternately spoon batters into prepared pan, beginning and ending with yellow batter.
BAKE 35 to 40 minutes or until wooden pick inserted in cake comes out clean. Cool in pan for 20 minutes; invert onto wire rack to cool completely. Dust with powdered sugar before serving.