Not to echo many others, but a small amount of chocolate is not usually fatal, or even dangerous, in most dogs. Some can have serious reactions. The type of chocolate does matter and how sensitive the particular dog is.
If you WANT to feed chocolate, use Carob, which is safe for dogs. They even make a "chocolate" cookie just for dogs with carob that look JUST like Oreos.
But chocolate CAN be dangerous. Although I have 2 dogs get into some chocolate and have no ill effects (Hershey's Kisses and EL Fudgesticks), another one had a serious GI upset and pancreatitis (fudge).
I would not want to give a treat that COULD be fatal to my dog. Give them something that won't harm them even in large amounts, in case they get into the bag when you are not looking, as they sometimes do. Mine would rather have a Greenie than a cookie any day. IF they get too many of those, they just poop loose green for a couple of days.