PREHEAT oven to 350°F. Line 8-inch baking pan with foil, with ends of foil extending over sides of pan; grease foil.
MICROWAVE chocolate and butter in large microwavable bowl on HIGH 2 minutes or until butter is melted. Stir until chocolate is completely melted. Add sugar; mix well. Blend in eggs and vanilla. Add cereal, baking powder and salt; mix well. Stir in walnuts. Spread into prepared pan.
BAKE 30 to 35 minutes or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan on wire rack. Lift brownie from pan onto cutting board, using foil handles. Cut into 16 squares to serve.