Yogurt is a good snack for dogs. If you want to use it to help their digestion, however, you need to make sure you're buying yogurt that has live acidophilus cultures in it, otherwise it's really nothing more than a pudding snack for them. The yogurt with the live cultures in it, however, is a good, healthy snack for them, in moderation, of course. A couple of tablespoons would be all I would give as a snack, or you could mix it in with their dry food.
We raise show dogs and we start our puppies on yougurt at 3 weeks of age and continuing feeding it. They love it and it is great calcium for them as puppies and adult dogs.
We feed yogurt to our dogs as well but we always get nonfat plain yogurt. I doubt that the amount of chocolate in your yogurt is enough to hurt your dog, but generally chocolate is not considered good for canines. We also make sure there's no sugar or preservatives in the yogurt. We usually buy nonfat Mountain High yogurt and we'll put it in their kongs with dry kibble and freeze it. They love it!