Line a baking sheet with parchment paper and set aside.
Combine butterscotch chips and peanut butter in a large microwave safe bowl. Microwave ingredients for 30 seconds, stir and repeat until completely melted.
Add the chow mein noodles to the peanut butter mixture. Stir until combined then add the marshmallows and stir to combine.
Drop by spoonfuls onto the prepared baking sheet.
Let cool at room temperature or place in the refrigerator or freezer to speed up the process.
Notes
If you store the haystack cookies in an airtight container they will keep for up to a week or longer at room temperature.Fridge: If your house is quite warm and it seems like your haystacks are softening at room temperature, you can keep them in the fridge for about a week.Freezer: Make sure haystacks are fully set and then place in a single layer in a freezer-safe container. For best results, separate them with layers of parchment or wax paper. This will keep them from sticking to each other.