In my honest opinion, I wouldn't go near the stuff! Not because coco is bad, but because I try to stick to the German purity law whenever I can. So what would I use?
Get a grain bag, add 250g of chocolate malt (crushed), tie the top and plunge into a pan of water at 66-77°C (no more, no less) for 30 minutes, then put on a sieve, sparge with another litre of water at 66-77°C, boil it up for 5 minutes to sterilise, add your tin of Irish Stout, pour into fermenter, top up to required volume with cold water and ferment as per usual. Good to give it 2 weeks in the fermenter untouched, then 2-4 weeks in the bottle before drinking.
If you do this you will DEFINITELY improve your stout beyond anything coco powder could do. You will not only add a chocolate taste but a genuine roasted barley flavour which kits I have tried have lacked. If its not chocolatey enough for you, next time just add more chocolate malt, say 300 - 400g?
This is a great way to take brewing to the next level for very little cash, and start brewing beers which are perfect for your taste. I personally find that far more satisfying than any single can kit can give me.