Brock describes how to made it:
1 or 2 bananas. One to make the hat, one to model the hat. This could also be made using one banana. Carve the shape of the hat using an x-acto knife. Leave one of the banana peel sides longer, to make the rim of the hat. Most bananas come with a little sticker. Use this sticker to serve as the logo on the hat, if you want your hat to have a logo.
Simple project, takes about 5 minutes yet the memories will last a lifetime.
via Brock Davis