In a large saucepan or high sided sauté pan add the butter, sugar, milk and hot chocolate mix.
Melt together and bring to a boil and boil for 1 minute then remove from heat.
Add in the marshmallow fluff and vanilla and whisk until smooth and combined.
Add in the oats and stir until full covered and coated.
Lay down a piece of parchment paper on a flat surface and using a cookie scoop or ice cream scoop, scoop about 2 heaping tablespoonfuls into mounds on the parchment paper and let set for about 20-30 minutes.
After setting, move to a wire rack to let the bottoms set for about 10-15 minutes, since this uses marshmallow fluff the bottoms need air to set up as well.
Notes
You need to use quick-cooking oats for these. If you use old-fashioned oats, the consistency won't be right and they'll be more difficult to eat.
You can use unsalted butter in this recipe as well. I think the salted butter helps keep these cookies from being way too sweet.
Regular cocoa powder can be used in place of the hot chocolate powder.
Store cookies in an airtight container. Place the cookies between sheets of wax paper to keep them from sticking together. The cookies can be stored for about a week out of the refrigerator, two weeks if stored in the refrigerator and about 2 months in the freezer.