Line an 8x8 pan with foil and spray with nonstick cooking spray. Set aside.
Pour marshmallows in a large, wide bowl. Set aside.
In a medium saucepan, melt the butterscotch chips, peanut butter and butter over medium heat, stirring constantly. Once the mixture is melted and smooth, remove from heat and let cool for 5 minutes, stirring occasionally.
Pour mixture over marshmallows, folding until marshmallows are completely coated. Pour mixture into prepared pan. Refrigerate until chilled and set, a couple of hours.