Lactose is definitely the way to go. I made a Mexican hot chocolate imperial stout (my only recipe on here) for my first batch of beer. That used cocoa nibs, cinnamon, ancho peppers, and vanilla beans as well as a half pound of lactose. The thing to remember is that unsweetened cocoa provides a bit of bitterness and an almost chalky quality on its own. I treated it almost as additional roasted malt or hop bitterness in my recipe that needed balancing. The vanilla also serves nicely to help round that out, many commercial chocolates contain vanillin.
Edit: Decide for yourself on the aging. I expected my stout to need several months of aging but honestly it tasted great within a week of bottling and I've struggled to hold on to any for aging (I've only had a handful of bottles but it seems popular...). But I also added my lactose during the boil, so that may change things.