PREHEAT oven to 350°F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted; cool to room temperature. Set aside.
BEAT eggs and sugar in large bowl with electric mixer on medium speed until thick and lemon colored. Blend in chocolate mixture. Add flour; mix well. Pour into greased and floured 9-inch round cake pan.
BAKE 30 min. or until center is set. Cool 5 min.; remove from pan. Cool completely on wire rack.