I have found that Everythingtea.net's Golden Monkey has that pronounced cocoa note, and I drink it almost every Saturday when I am working there. That being said, I work there so my opinion should be taken with a horse-lick if I am not actually violating some rule somewhere about posting and advertising. I did give full disclosure in any case.
On the cocoa side of thing's I recommend trying your favorite dark chocolate with your black tea, the two are wedded to each other, especially the Chinese black teas. Hao Ya A is always my favorite for the chocolate pairing, but the congou class is actually where I first started pairing the two and I go back to that happy place often.