HEAT oven to 350°F.
LINE 9-inch square pan with foil, with ends of foil extending over sides; spray with cooking spray.
MIX flour, baking powder and salt until blended; set aside. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted; stir until chocolate is completely melted and mixture is well blended. Add sugar; mix well. Blend in eggs and extract. Add flour mixture; mix well. Stir in nuts. Spread onto bottom of prepared pan.
BAKE 30 to 35 min. or until toothpick inserted in center comes out clean. Cool completely. Use foil handles to lift dessert from pan before cutting into bars.