I'll second that loading it up with some more chocolate malt will get you better results over cocoa powder, however...
I have used chocolate in 2 stouts and 1 porter, but I only used powdered cocoa once, and had the worst results of the three.
I remember doing sort of a mini-decoction with a small part of the wort, added the cocoa powder, and brought it up to a boil. I then scrapped off all the oils that floated to the stop, then reduced heat. I kept this warm on a burner so it wouldn't solidify, and then added it back in with the rest of the wort at the end of the boil.
I used a mix of plain old Baker's dark chocolate and Ghiradeli baking choclolate and had much better results than using powder.
In all three cases, the final result did have a nice, dusty, chocolately finish, the small amount of real chocolate added just gave the chocolate malt a nice smooth kick. I think it helped a bit with head retention too, nice, creamy head.