PREHEAT oven to 350° F. Grease 9-inch-square baking pan.
COMBINE flour, cocoa, baking powder and salt in medium bowl. Beat sugar, butter and vanilla extract in large mixer bowl. Add eggs, one at time, beating well after each addition. Gradually beat in flour mixture. Stir in 1 1/2 cups morsels. Spread into prepared pan.
BAKE for 25 to 30 minutes or until wooden pick inserted near center comes out slightly sticky. Cool in pan on wire rack (brownies will sink in center when cooled).
MICROWAVE remaining morsels in heavy-duty plastic bag on MEDIUM-HIGH (70%) power for 30 to 45 seconds; knead. If necessary, microwave at additional 10- to 15-second intervals, kneading until melted. Cut tiny corner from bag; squeeze to drizzle over brownies. Refrigerate for 5 minutes. Cut into squares.