Preheat oven to 350 degrees.
Melt butter and baking bars in heavy saucepan over low heat, stirring until smooth.
Cool to room temperature.
Stir in 1 3/4 cups sugar.
Beat in 3 eggs; stir in flour.
Spread into greased 13 x 9 inch baking pan.
Beat cream cheese and remaining 1/2 cup sugar in small mixer bowl.
Beat in remaining egg and vanilla extract.
Pour over chocolatem mixture.
Deeply swirl batters with knife.
Bake for 30 to 35 minutes or until toothpick inserted in center comes out slightly sticky.
Cool completely in pan on wire rack.