Line 8-inch square pan with foil, with ends of foil extending over sides of pan. Grease foil; set aside.
Microwave chocolate and butter in large microwaveable bowl on HIGH 1-1/2 minute or until butter is melted. Stir until chocolate is completely melted. Stir in sugar. Blend in eggs. Add flour, baking powder and salt; mix well. Stir in nuts.
SPREAD into prepared pan.
BAKE 30 to 35 minute or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan. Lift out of pan onto cutting board. Carefully remove brownie from foil. Cut into 16 squares to serve
Note: To Double
Prepare batter as directed, doubling all ingredients. Pour into 13x9-inch foil-lined baking pan. Bake in preheated 350°F-oven 40 to 45 minute or until wooden toothpick inserted in center comes out with fudgy crumbs. Remove brownie from pan, using foil handles. Cool, then cut into 32 squares to serve. Makes 32 servings, 1 brownie square each.