Microwave chocolate and butter in large microwaveable bowl on MEDIUM 2 min. or until butter is melted. Stir until chocolate is completely melted and mixture is well blended. Stir in 1-1/2 cups sugar. Add 4 eggs, 1 at a time, mixing well after each addition. Stir in 1-1/2 cups flour just until blended. Spread onto bottom of 13x9-inch pan sprayed with cooking spray.