PREHEAT oven to 350° F. Grease 13 x 9-inch baking pan.
COMBINE butter and Choco Bake in a medium, heavy-duty saucepan over lowest possible heat, stirring until smooth. Cool to room temperature. Combine flour, baking soda and salt in small bowl. Stir 1 3/4 cups sugar into Choco Bake mixture. Beat in 3 eggs; stir in flour mixture. Spread into prepared baking pan (batter will be thick).
BEAT cream cheese and remaining 1/2 cup sugar in small mixer bowl. Beat in remaining egg and vanilla extract.
DROP tablespoons of cream cheese mixture randomly over chocolate mixture. Swirl mixtures deeply with spoon, pulling chocolate mixture up to surface.
BAKE for 25 to 30 minutes or until wooden pick inserted near center comes out slightly sticky. Cool completely in pan on wire rack. Refrigerate for 1 hour. Cut into bars.