HEAT oven to 350°F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until melted. Stir in sugar. Add eggs and vanilla; mix well. Stir in flour and nuts.
SPREAD into greased 13x9-inch pan.
BAKE 30 to 35 min. or until wooden toothpick inserted in centre comes out clean. (Do not overbake.)