Preheat oven to 350°F. Line 13x9-inch baking pan with foil, with ends of foil extending over sides of pan. Grease foil. Microwave unsweetened chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Stir in sugar. Add eggs and vanilla; mix well. Stir in flour until well blended. Spread into prepared pan.