PREHEAT oven to 375° F. Grease 10-cup Bundt or round tube pan.
MICROWAVE 1 1/3 cups morsels in medium, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 1 minute; STIR. The 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, bananas, 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 morsels. Alternately spoon batters into prepared baking pan.
BAKE for 35 to 45 minutes or until wooden pick inserted in cake comes out clean. Cool in pan for 20 minutes; invert onto wire rack to cool completely.
PLACE remaining morsels in small, heavy-duty plastic bag. Microwave on MEDIUM-HIGH (70%) power for 30 seconds; knead. Microwave at additional 10- to 15-second intervals, kneading until smooth. Cut tiny corner from bag; squeeze to drizzle over cake.