Line 9-inch square pan with foil, with ends of foil extending over sides; grease foil. Microwave semi-sweet chocolate and 1-1/2 cups milk in medium microwaveable bowl on HIGH 2 min., stirring after 1 min. Stir until chocolate is completely melted and mixture is well blended. Add vanilla and nuts; mix well. Pour into prepared pan. Refrigerate while preparing the top layer.