Wow, for a one-off event you're clearly quite uncomfortable with, you're getting a heck of a flaming.
We all know it's not acceptable to hit our dogs. The OP knows this; she's said she feels awful. It's happened, it shouldn't have happened, but it has happened; let's accept that and deal with the more important things here.
1 -If it was a one-off incident and you didn't immediately smother the dog with reassurance, your dog will most likely not make a big deal out of this incident. In packs dogs will snap at each other to put one another in their place. Just remember how awful you feel now and you won't do it again.
2 - Your dog has eaten chocolate. It's fairly likely everything will be ok, but to be on the safe side call the vet (if you haven't already done so), explain the situation and the dog's size, the amount of chocolate etc and figure out what to do.
3 - More long term here, but you need to address your dog's food thievery. Yes most dogs seem to be perpetually hungry, but this is something which can be managed with time and training. If you can work on commands like 'stay' and 'leave', prevent jumping up if that's a problem, get the dog into the habit of always eating from his bowl, and only hand feed rewards when he's done something to earn the reward then you might be able to combat this behaviour. It's not quick or easy, but it's very possible to have a dog and not live in fear that the snack you left on the coffee table will immediately be scoffed!