In a large pot, over low heat, melt the butter and peanut butter together. Stir until smooth and melted.
Add in butterscotch chips and stir until melted. Mixture should be smooth.
Remove from heat and allow to cool for 5 to 10 minutes.
Add marshmallows, a little at a time, and stir thoroughly, but carefully, you don't want to melt the marshmallows
Pour into a parchment lined 13x9 pan and refrigerate until cold and firm.
Cut into squares and store in the fridge in an airtight container.