Line a 9-in. square pan with foil and grease the foil with butter; set aside. In a microwave, melt frosting and chocolate chips; stir until smooth. Stir in walnuts; cool for 10 minutes. Stir in marshmallows. Transfer to prepared pan. Cover and refrigerate until firm. , Using foil, lift fudge out of pan. Discard foil; cut fudge into 1-in. squares. Store in an airtight container in the refrigerator.