PREHEAT oven to 350°F. Line 13x9-inch baking pan with foil, with ends of foil extending over sides of pan. 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.
STIR sugar into chocolate until well blended. Mix in eggs and vanilla. Stir in flour and nuts. Spread in greased foil-lined 13 x 9-inch baking pan.
BAKE 30 to 35 min. or until wooden toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan on wire rack. Remove brownies from pan, using foil handles. Cut into squares. Store in tightly covered container at room temperature.