Awww, poor baby! He probably thought he deserved chocolate!!! Seriously, I hope he'll be OK. He should be fine but I wiould watch him for the "signs" vets suggest. Here are two pages listing what to watch fir: (But you know your kitty so will know what is normal):
http://vetmedicine.about.com/cs/nutr...latetoxici.htm
http://www.pets.ca/pettips/tips-20.htm
Give him a hug and a petting for me (and a hug for yourself from me too!!)
Good luck!!!