In 3- to 4-quart saucepan, melt 1 cup butter and 4 oz chocolate over very low heat, stirring constantly. Remove from heat; cool slightly. Stir in 2 cups sugar and 2 teaspoons vanilla. Add eggs 1 at a time, beating well after each addition. Stir in flour; mix well. Spread in pan. Carefully spoon cheese filling over brownie mixture. Gently cut through layers with knife to marble.