Line 13x9-inch pan with foil, with ends of foil extending over sides; spray with cooking spray. Microwave unsweetened 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 granulated sugar; mix well. Blend in eggs and 1 tsp. vanilla. Stir in flour until blended. Pour into prepared pan.