PREHEAT oven to 350°F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until melted. Stir in sugar until well blended; mix in eggs and vanilla. Stir in flour and nuts.
SPREAD mixture in lightly greased 13x9-inch baking pan.
BAKE 30 to 35 min. or until wooden toothpick inserted in centre comes out clean. DO NOT OVERBAKE.