HEAT oven to 350°F. Line 9-inch square pan with foil, with ends of foil extending over sides; grease foil.
MICROWAVE chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Add sugar; mix well. Blend in eggs and extract. Add flour, baking powder and salt; mix well. Stir in nuts. Spread into prepared pan.
BAKE 30 to 35 min. or toothpick inserted in center comes out clean. Cool in pan on wire rack. Use foil handles to lift dessert from pan before cutting to serve.