Combine first 2 ingredients. Melt in microwave; stir until smooth. Chill in refrigerator. Combine next 3 ingredients; set aside. Beat egg whites and cream of tartar until firm peaks form. Fold egg white mixture into brownie batter. Place batter in greased 8 x 8 baking pan; swirl chilled chocolate mixture into brownie batter. Bake at 375° for about 15 minutes. Do not over bake. Cool completely before serving. Makes 8-10 servings.