In a Medium saucepan melt together butter, peanut butter and butterscotch chips on very low heat until dissolved. Stir constantly to avoid burning. Remove from heat and allow to cool a bit before adding marshmallows (so they won't melt).

 

Pour into a 11" by 13" pan or a 9" by 9" pan (if you prefer thicker squares) and refrigerate for an hour to set. Cut into squares and serve.