PREHEAT oven to 375° F. Grease 13 x 9-inch baking pan.
MICROWAVE 1 cup 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. Cool to room temperature. Combine flour and baking powder in small bowl.
BEAT sugar, butter and vanilla in large mixer bowl until crumbly. Beat in eggs. Add melted chocolate; beat until smooth. Gradually beat in flour mixture. Spread batter into prepared baking pan.
BAKE for 16 to 20 minutes or until wooden pick inserted in center comes out slightly sticky.
REMOVE from oven; sprinkle immediately with marshmallows, nuts and remaining morsels. Return to oven for 2 minutes or just until marshmallows begin to melt. Cool in pan on wire rack for 20 to 30 minutes. Cut into bars with wet knife. Serve warm.