Line 8-inch square pan with foil, with ends of foil extending over sides. Microwave semi-sweet chocolate and 1/2 cup milk in medium microwaveable bowl on HIGH 1-1/2 min. or until chocolate is almost melted, stirring after 45 sec. Stir until chocolate is completely melted and mixture is well blended. Stir in vanilla.
Spread onto bottom of prepared pan.
Microwave white chocolate with remaining milk as directed for semi-sweet chocolate; pour over semi-sweet chocolate layer in pan. Swirl gently with knife.
Refrigerate 2 hours or until firm. Use foil handles to remove fudge from pan before cutting into pieces.