Chocolate wafer, or any kind of wafer or chocolate is not suitable for bunnies and please do not feed it to your bunny again.
Do you have your rabbit registered at a vet? You may need one.
The most important thing now is to get your rabbit eating lots of hay. The sugar could very easily upset the rabbits tummy and unlike in humans this can be very very serious. You need to monitor what your bun eats/drinks and most importantly poops over the next 48 hours. If there is any change from normal and if the rabbit seems off colour in any way I would say a vet visit would be best.
If the bun stops eating and pooping it is called stasis (where the gut stops moving things along) and stasis can kill in hours rather than days and the rabbit would need a vet as a matter of urgency.
You might want to phone a vet anyway and get some advice.