Where a young mother of a (super adorable) one-year-old finds the time to do this sort of thing, I don't know. But she did, and it was a very enjoyable Christmas offering.
My only tip would be, place the marshmallows within the areas you intend to cut, so your knife doesn't come in contact with the gooeyness. It made cutting them a bit of a massacre, so there are no pictures of the finished brownies. But... yummmmm.