HEAT oven to 350°F.
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 vanilla. Stir in flour and nuts.
SPREAD into 13x9-inch pan sprayed with cooking spray.
BAKE 30 to 35 min. or until toothpick inserted in centre comes out clean. (Do not overbake.)