These are edible, chocolate...party hats. You wear them when, well...you and your little buddy want to party. As you can see, they're already available in two versions: the viking, and the cowboy. But soon, there'll be a third version: UNSC Spartan John-117. Not the most official piece of Halo merchandise, then, but it's probably the naughtiest.
Chocolate Relations [thanks Mdawg!]