Line a 9x9-inch baking pan with aluminum foil and spray with nonstick spray and set aside.
Combine the butter, butterscotch chips, and peanut butter in a medium saucepan and cook over medium heat just until melted and smooth.
Remove from heat and fold in 3 cups of the marshmallows. Pour the mixture into the prepared baking pan, top with the remaining marshmallows and allow to set.
Cut into squares and enjoy. Store in an airtight container for up to 2 weeks in the refrigerator.