Chocolate is very, very toxic to rabbits. I know of a bunny who needed charcoal due to eating a lot of chocolate in a similar situation. It would be sensible to contact your vet for advice. It would also be worthwhile watching for any signs of problems (which i'm sure you are) but also add to that anything neurological, like scanning eyes, or different movements, or anything at all that is not right. If any of those things happen you need to seek vet treatment immediately.
Bunnies are both hilarious and stressful all in one moment.