I make a chocolate Raspberry imperial stout and with that strength it was initially hard to get both the flavor and aroma to come thru. My LHBS pointed me to the fruit flavorings section where there is a chocolate extract specifically for brewing (LD Carson or Brewers best brand).
It is not cheap but is concentrated enough that a 4oz bottle is enough for a normal 5G batch. If you put it in just before bottling/kegging, then mix gently, it will carry through very nicely to the final serving.
I have tried other methods like cocoa and nibs, but they tend to cloud the final product or the flavor is lost with whatever fining/filtering is done.
If you search the web, there are also several tutorials on making your own extract with nibs/cocoa and vodka to extract it. The extract is filtered so there is no fat to worry about. There seems to be a sweet-spot in the vodka-water ratio used, so read up on a few sites before you try it.