In a saucepan over low to medium heat, melt the butter, butterscotch chips and peanut butter together, stirring constantly. Do not let the mixture boil, all you want to do is melt everything.
Let the mix cool for a few minutes before you add the marshmallows, so you don't melt them.
Mix thoroughly.
Then transfer to an 8 x 8 " square dish.
Refrigerate, and serve when cooled completely.
This square is best kept in the fridge between snacking!
Notes
You could easily cut this recipe in half. I do this for our family of 5, and there is plenty!