Ok, I've been watching this now for a while and I am forced to add my .02. I read somewhere (old brewing wives tale) that something in cocoa inhibits yeast performance in some way. I tried cocoa (always the Hersheys baking cocoa powder, baking cocoa powder is defatted, the bars are not) in the last 5 min of the boil. 1 oz by weight per 5 gal. It was ok. I then tried varying amounts in secondary, wetted by vodka so it would dissipate without clumping.
The base recipe was always a mocha porter, so I was looking for more of a subtle flavor to complement the roasted barley. The best has been - 2 oz by weight in secondary for 7 days. I guess the vodka helps extract the flavor. The effect was a very smooth cocoa flavor, but it did leave a lot of crap in the keg. Very popular at Xmas time.