Heat oven to 350°F. Line 13x9-inch 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 vanilla. Stir in flour and nuts until well blended; spread into prepared pan.