Grease a 9x13-inch pan.

Place butterscotch chips, peanut butter, and butter in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until melted and combined, about 5 minutes.

Stir marshmallows, coconut, and walnuts into butterscotch mixture until evenly combined.

Pour mixture into prepared pan. Refrigerate until set, about 1 hour. Cut into 18 squares and store in the refrigerator.