PREHEAT oven to 350º F. Grease 9-inch-square baking pan.
MICROWAVE chocolate in medium, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 1 minute; STIR. If pieces retain some of their original shape, microwave at additional 10- to 15-second intervals, stirring just until melted; cool to room temperature.
BEAT cream cheese and 1/4 cup sugar in small mixer bowl until smooth. Beat in 1 egg and 1/2 teaspoon vanilla extract.
COMBINE flour and salt in small bowl. Beat remaining 3/4 cup sugar and butter in medium mixer bowl until creamy. Add remaining 3 eggs, milk and remaining teaspoon vanilla extract; beat until well mixed. Beat in melted chocolate. Stir in flour mixture until blended. Spread into prepared baking pan.
DROP tablespoons of cream cheese mixture randomly over chocolate mixture. Swirl mixtures deeply with spoon, pulling chocolate mixture up to surface.
BAKE for 33 to 38 minutes or until wooden pick inserted in center comes out slightly sticky. Cool completely in pan on wire rack. Refrigerate for 1 hour. Cut into bars.