Line 13x9-inch pan with foil, with ends of foil extending over sides; spray with cooking spray. Microwave unsweetened chocolate and 3/4 cup 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 2 cups sugar; mix well. Blend in 3 eggs and vanilla. Add 1 cup flour and cinnamon; mix well. Pour into prepared pan.