PREHEAT oven to 350º F. Grease 9-inch-square baking pan.
MICROWAVE 1 cup morsels in small, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 45 seconds; STIR. If necessary, microwave at additional 10- to 15-second intervals, stirring just until smooth. Cool to room temperature.
COMBINE flour, whole-wheat flour, baking powder and salt in small bowl. Beat brown sugar and butter in medium mixer bowl until creamy. Add eggs and vanilla extract; beat well. Beat in melted chocolate. Gradually beat in flour mixture. Stir in bananas and remaining 3/4 cup morsels. Spread into prepared baking pan.
BAKE for 40 to 45 minutes or until wooden pick inserted in center comes out clean. Cool completely in pan on wire rack. Cut into bars. Store in covered container in refrigerator.