Most Chocolate Stouts don't actually have chocolate, but use roasted malt that tends to have a bitter chocolate flavor.
There are a few beers that do use real chocolate with mixed results. If there was anything like this at WDW world, as others mentioned, my guess is that it was Sam Adams' Chocolate Bock. It's a winter seasonal that's available in bottles in the Winter variety pack. I've had it on tap and it definitely has a real cocoa taste. It was very interesting for the initial tastes, but became cloying by the time I was finishing the pint. I felt the Chocolate Bock worked best as a rare oddity, and could never imagine drinking more than one pint in a row.