Line a 9x9 square pan with foil, overlapping the sides for easy fudge removal after refrigerating. Grease foil to prevent sticking.
In a large microwave safe dish, pour in sweetened condensed milk. Place butter and chocolate chips on top of milk. Microwave in 30 second increments, stirring in between, until mixture is melted, smooth, and completely combined. Be careful not to scorch (mine took less than 2 minutes). Fold in marshmallows, walnuts, and cherries.
Pour into the foil-lined pan and spread evenly. Refrigerate until firm, about 4 hours. Lift foil from pan. Using a long, sharp knife, cut into 1inch cubes, or to your liking.