I've done much research and personal experimentation. One method that, for me, achieved satisfactory results was dissolving 3 oz (weight) Tollhouse cocoa powder into 10 oz (volume) of vodka and adding that to the secondary at racking. It makes a royal mess, but the chocolate flavor and nose is preserved, and the cocoa itself is sterilized.
Adding cocoa to the boil is inefficient, in the sense that the stuff won't stay in suspension, as there is no alcohol yet to keep it dissolved. To see this for yourself, make a cup of hot cocoa and let it sit for about a week. You end up with water and chocolate sludge on the bottom of your cup. In other words, also a royal mess where brewing is concerned, as 80% of your cocoa ends up in a gelatinous heap at the bottom of the primary. As an aside, the notes about high SG might be because a property of the cocoa itself is inhibiting the yeast. Just food for thought.
Either way, I think the best and most painless method is the addition of chocolate essence at bottling/kegging.
Nielsen Massey and
Star Kay White are the only brands I'm aware of that make top quality chocolate essences, which do not include sugar (table, corn, and/or agave) as a major component. In other words, they're the pure, real deal. Nielsen Massey is widely available; check the dealer finder on their web page. One 4 oz bottle should do the trick.
Adding some measure of vanilla extract at bottling in addition to the chocolate essence -- about 2 oz -- is a good way to make the chocolate flavor pop.
Vanilla from Tahiti makes the best unsweetened extract out there, and the flavor of Tahitian beans goes very well with beer. Any pure vanilla extract (ie: not artificial) will do in a pinch.
Note that neither the chocolate or vanilla extracts should be boiled. They're 40% alcohol and sterile; a boil will completely ruin them.
The disclaimer I have to offer is that I haven't yet gone the essence route myself, but am about to use it in a stout I'm bottling in a few days. I'm pretty passionate about finding the ideal way to add chocolate to a brew, and after experimenting with various methods I think the essence is probably the easiest and most effective.
We'll see. If I'm not getting any chocolate nose/flavor with 4oz choco + 2 oz vanilla then this route would not be entirely cost effective (ie: if I have to add more than the combined 6 oz it would start getting prohibitively expensive).