HEAT 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.
BEAT eggs and sugar in large bowl with mixer until well blended. Whisk in chocolate mixture. Stir in flour until well blended. Pour into greased and floured 9-inch round pan.
BAKE 30 min. or until center is set. Cool 5 min.; remove from pan to wire rack. Cool completely.