First, melt the butterscotch chips and peanut butter in a medium saucepan over low heat, stirring just until barely melted and smooth. Remove from heat and add the chow mein noodles.
Wait 1-2 minutes before stirring in the marshmallows - you don't want them to melt so if your mixture is really hot, wait until it cools a bit before adding the marshmallows. Your peanut butter mixture shouldn't be very hot, but if you heated it a little bit too long, give it time to cool.
Use two spoons to drop mixture by cookie size onto wax paper to cool. The setting process will take a few hours at room temperature but takes a lot less time if you put the cookies in the fridge to set.