Line 9-inch square pan with foil, with ends of foil extending over sides; spray with cooking spray. 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. Stir in 2/3 cup sugar. Add 2 eggs and vanilla; mix well. Stir in 1 cup flour and 1/3 cup nuts. Pour into prepared pan.