Add salted butter to a large nonstick pot over medium heat. Do not throw the butter wrapper away - set it aside for later.
Stir the butter until melted.
Once melted, it will start to foam up and turn clear-ish - at this point, make sure to keep stirring and watching as the butter begins to brown.
After a few minutes, it will start to smell nutty and the color will turn to a warm brown color.
At this point, you’ll want to add the marshmallows - reserving a handful for later.
Stir in the marshmallows and turn the heat to low. Once they're melted and combined, stir in the cheerios. Do not overcook the marshmallows because they'll get rock hard.
Quickly mix everything together until all the cheerios are coated in the brown butter marshmallow mixture. Then add in the reserved handful of marshmallows and stir once more.
Add a sheet of crumbled parchment paper to a baking dish then rub the paper that the butter came in along the parchment paper to help the treats from sticking.
Quickly and carefully pour the treat mixture into the lined baking dish.
Next, grease your hands with the butter paper and carefully (if not too hot) pat down the treat mixture so it has a flat, even surface.
Let cook for a few minutes before digging in.