Line a 9-in. square baking pan with foil. Grease foil with butter; set aside.
Combine milk and chips in a large microwave-safe bowl. Microwave, uncovered, on high for 1 minute; stir. Cook 30-60 seconds longer, stirring every 30 seconds, or until chips are melted. Stir in candy bars, M&M's and vanilla. Transfer to prepared pan. Cover and refrigerate for 2 hours or until firm.
Lift fudge out of pan using foil. Gently peel off foil; cut fudge into 3-inch rows. Cut each row into eight triangles. Gently press M&M's minis into fudge for eyes and mouth; press in pretzels for antlers. Store in an airtight container.