The only chocolate flavour I tried isn't intended for 'e-cigs' - it's a chocolate 'tobacco flavouring' I got from from 'Ali Bongo' (.com) - I use quite a few of these (Tasty Puff is one brand, about £3.25 per 10ml bottle) flavourings for my DIY and they seem to work great - the chocolate one is really strong though, and I got told off for making the whole house stink of it when I used it at 5%, and it drove the dog utterly nuts as well, so I dropped it to about 2% and that seems plenty.
I also add a bit of vanilla too (I use pure vanillin dissolved in hot VG), and a tiny bit of home made coffee flavour as well and this gives me a great flavour.
Cheers!
DV