I just read an article about a rabbit owner who had three of their rabbits die from eating chocolate, so I’d have to say that it probably isn’t a good idea to give your bunny chocolate, even in small amounts. Now the above bunnies got into a box of candy, and probably ate way more than your bunny did, but it’s still better to be safe than sorry.
Bunnies also naturally have an extremely low fat diet, so feeding your bun extra fat is not good for it.
I have had many pet bunnies over the years, and the following were always very successful as treats:

Uncooked oatmeal (all my bunnies acted like this was crack. Give in small amounts, though. It’s got a lot of fiber.)
Strawberries
Ice cream (a low fat soy product would be healthier than cow’s milk)
Apple peel, apple pieces
Watermelon (I had one bunny that was wild over watermelon)
Grass, clover, shepard’s purse, dandelion leaves (make sure it comes from fields and lawns that are not chemically treated, of course)
Sunflower seeds (in small amounts due to high fat content)

Fruit and greens should be used often but in small amounts, as too much can give a bunny the runs. (not a pretty sight!) Grains should be given in small amounts because they can have the opposite effect. And it’s better to give your bunny natural treats rather than those seed and grain sticks they sell in the shops. Those things have way too much added sugar, and your bunny will like his grains and such just fine without it.

Hope that helps!