Line a 9-in. square pan with foil; grease foil with 1-1/2 teaspoons butter., In a large microwave-safe bowl, melt chocolate chips and remaining butter, stirring after 1 minute and every 30 seconds thereafter. Stir in milk and vanilla. Spread into prepared pan. Refrigerate until firm., Using foil, lift fudge out of pan. Remove foil; cut fudge into 1-in. squares. Store in an airtight container in the refrigerator.