Line 8-inch square pan with foil, with ends of foil extending over sides. Spray with cooking spray. Microwave chocolate and butter in large microwaveable bowl on HIGH 1-1/2 min. or until butter is melted. Stir until chocolate is completely melted. Add sugar; mix well. Blend in eggs. Add flour, baking powder and salt; mix well. Stir in nuts.
Spread into prepared pan.
Bake 30 to 35 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan. Use foil handles to lift brownies from pan before cutting to serve.
Recipe can be doubled. Prepare batter as directed, doubling all ingredients and baking in foil-lined 13x9-inch pan 40 to 45 min. or until toothpick inserted in center comes out with fudgy crumbs. Makes 32 servings.
Cut brownie into assorted shapes with cookie cutters. Then, let the kids help decorate the cutouts with decorating gels and sprinkles for family fun.