Line 8x8-inch pan with parchment, with ends extending over sides. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted and smooth. Whisk in 2 eggs, 1/2 cup sugar, 1/2 tsp. vanilla and flour until blended. Spread onto bottom of pan. Set aside.