Heat a large pot over low heat and mix together peanut butter, butter and butterscotch chips. Stir constantly until completely melted and smooth. Remove from heat and set aside to cool for 10 minutes. Fold in marshmallows until evenly coated. Spray a 9x13-inch baking dish with nonstick cooking spray then spread mixture in the pan. Let sit at room temperature or in the fridge until firm.