I've done a triple chocolate stout. I used dark cocoa powder at the end of the boil / knockout as well as chocolate extract at bottling along with the standard chocolate malts, etc. I also used vodka to extract more chocolate flavor and aroma from cocoa powder and added that to the bottling bucket, as well. Even accounting for the bitterness of cocoa powder and hops, it is massively sweet. Of course, my OG was over 1.1 and my FG was only around 1.03 or so.