Step 1

Preheat oven to 375° F. Grease 8- or 9-inch loaf pan.

Step 2

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.

Step 3

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.

Step 4

Stir 2 cups batter into melted chocolate. Alternately spoon batters into prepared pan, beginning and ending with yellow batter.

Step 5

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.