Place the bag of mini marshmallows into the freezer while you prep everything else (15-20 minutes).
Line a 9x13 baking pan with parchment paper.
In a saucepan, combine butter, butterscotch chips, and peanut butter. Heat on medium-low, stirring, until smooth and creamy and the butter is melted. Do NOT bring to a boil.
Pour the cold marshmallows into a large bowl. Pour peanut butter/butterscotch mixture over.
Stir until completely coated.
Pour into prepared baking pan and pat with a rubber spatula to evenly distribute.
Chill in the refrigerator for 1-2 hours.
Cut into 24 squares.