A 'Halloween Mummy' is a mummy that is specific to Halloween. That is, rather than being an actual 'mummified' anything, a Halloween Mummy is really just a nod to a 'spooktacular' time of the year. A Halloween mummy can be a decoration, costume, or even part of clothing. In the case of this recipe how-to, a Halloween is basically a baked brownie that is decorated to look like a mummy.