Preheat oven to 350°.
Beat sugar and margarine with a mixer at medium speed until light and fluffy. Add egg, egg white, and vanilla; beat well. Gradually add flour and cocoa, beating well. Pour into an 8-inch square pan coated with cooking spray.
Beat cream cheese and sweetener with a mixer at high speed until smooth. Add milk; beat well. Pour cream cheese mixture over chocolate mixture; swirl together using the tip of a knife to create a marbled effect.
Bake at 350° for 30 minutes. Cool completely in pan on a wire rack. Cut into squares.
Tip: Don't use reduced-calorie or fat-free tub margarine in this recipe because those products contain water, which will make the brownies gummy.