In a glass bowl, heat butterscotch chips, peanut butter, and butter. I like to microwave it for a minute or so, until the chips are about half melted. Stir until everything is melted, combined, and smooth.
Add in mini marshmallows, stir until all are evenly coated. Use between 1/2 and 1 small bag of them, depending on your tastes. I usually use about 2/3 of a bag, but used a whole bag for this batch (as pictured). Sometimes you want more of the “fudge” part, sometimes you want more marshmallows. These bars are NOT a hard science 🙂
When marshmallows are coated, spread mixture into a lightly greased pan. (I used 8″ x 8″ for 2″ thick bars, 9″ x 13″ is a popular size to use for thinner bars.) Chill until set.
Slice up and serve!