I would add 3-4oz good cocoa powder in the last 5-10 min of the boil, then add cold steeped coffee at bottling to taste. Personally, I would do it with a porter. I think you could get a better mocha flavor than with a stout. Taste a sample before you bottle. If it isn't enough chocolate, you could always add nibs in secondary, but have had great results with just Ghirrardelli cocoa powder in my chocolate porter.