I'm not positive on this, but I think I remember hearing Jamil Zainasheff (mrmalty.com) at some point on the Sunday Session (The Brewing Network [thebrewingnetwork.com]) saying he used unsweetened (or regular, can't remember) Hershey's cocoa powder. His point was that in chocolate bars, syrups, and the like, you get large doses of adjuncts (milk, gluten, etc.).
I think the closer you get to pure cocoa, the more that spicey, chocolaty taste will come through.
On the other hand
http://www.homebrewtalk.com/f12/recipe-socias-black-chocolate-robust-porter-ag-157402/
He used some gherideli and the recipe looked good.
I'll take a look at brewing classic styles a little later and let you know if there's anything about adding chocolate.
Best of luck. Post your recipe in the recipe forum when you're done, and update with how it turned out.