I have put 8-12oz bakers cocoa powder in the last couple minutes of the boil. Using sweet chocolate or a chocolate bar makes it a pain with all the fat that floats. That won't make it sweet, just have a nice chocolate after taste, like double chocolate stouts. To make it sweet, add lactose. I did the Jamil choc hazelnut porter from B3 and loved it. Next time I would do a little less crystal and a little more chocolate. It had a little bit of a sweet "edge" to it. I would rather have it be more creamy....hmmmm..lactose maybe. Hope this helps!
**seefresh beat me to it**