Before coming to the US I didn't even know chocolate was bad for dogs. The dog I had at the time (medium size shepherd mix) ate a lot of the same things we did, as treats, pieces of dark chocolate, cookies, I'd give him my empty bowl of chocolate ice cream / pudding to lick clean etc. etc. He never was sick, and lived to be 14 and stayed in pretty good health.
I'm sure your dog will be fine.
Earlier this year Sunshine ate almost an entire, fresh-from-the-oven Strawberry Cream Cheese Butter Braid. If you've never had one of these pastries they are very large (about 8" x 20" after baking), very buttery/rich AND pretty expensive ($12). I was more angry about losing my breakfast and my money than I was about the dog giving herself a rotten tummy! That DID reappear in a lovely hot-pink mess, all over my beige carpet... :-(