Prepare the muffin mix as directed on the package.
Immediately after the muffins are fully baked, while still in the muffin tin, top each muffin with a single layer of marshmallows.
Place the muffin tin back in the oven under the broiler for 30-60 seconds until the marshmallows are lightly toasted. (Don't walk away- this will happen quickly.)
Notes
If you have trouble removing the muffins with the marshmallow topping, spray a knife with cooking spray before loosening them from the pan.