If your wine is still fermenting and under airlock, and if you are adding chocolate in solid form try to find a dark semisweet or bittersweet chocolate with a high percentage of cacao...Scharffen Berger Chocolate Bar, Extra Dark (82% Cacao) is just one example. You can buy it by the bar and just break it up and drop it into your carboy. Or you can find "pastilles" which are chocolate drops. I have added 3 oz per gallon over a period of 2 months or longer.
If you are bulk aging and want to add chocolate impact, then you should probably try a quality chocolate extract, like Star Kay White brand (Amazon, eBay, OliveNation). Bench trial to find what you like---fabulous stuff!!
Of course if you know you want to go chocolate when you start your wine, I would add a quality Dutch processed unsweetened cocoa powder, 4 oz by weight, per gallon. It will create thick fudgy lees and likely take 2+ rackings before the gross lees settle out...but boy does it add a kiss of chocolate to your wine. Not like the nasty chocolate wines in liquor stores.