Line a 9×9-inch pan with aluminum foil and spray with cooking spray; set aside.
To a large pot, add the unsalted butter, creamy peanut butter, butterscotch chips, and salt. Heat over low heat to melt, stirring nearly continuously until melted. Be careful so you don’t scorch the butterscotch chips. Starting with softened butter is very helpful. If the last few butterscotch chips are resistant to melting, it’s okay.
Remove pot from the heat, add the mini marshmallows, reserving 1/2 cup for sprinkling on top, and stir to combine. Work quickly and don’t overmix because you don’t want all the marshmallows to melt; some will melt and some will stay intact.
Turn mixture out into prepared pan, smoothing the top lightly with a spatula.
Evenly sprinkle with reserved 1/2 cup marshmallows, tapping them down gently with your fingers or a spatula.
Refrigerate for at least 4 hours or overnight before slicing and serving. Enjoy directly from the fridge. Fudge will keep airtight in the fridge for up to 1 month.
