Most likely nothing will happen except a little diarrhea as there probably was not enough chocolate to hurt your dog.
For future reference, here are instructions on what you should do if your dog does eat chocolate.
Different kinds of chocolates have different toxicity amounts. Chocolate contains theobromine, which is what causes the toxic response. If a dog ate the chocolate recently (within last 2 hours), vomiting should be induced with 1 teaspoon of hydrogen peroxide 3% per 10 pounds, which can be repeated 10 minutes later if it does not induce vomiting. Items such as cookies and cakes will have less actual chocolate in them, so you will have to make an estimate in the amount of chocolate that may have been in the amount of cake, brownie, or cookie your dog may have ingested .
Signs of theobromine toxic poisoning should appear within 1 to 4 hours of ingesting the chocolate. These include vomiting, increased thirst, diarrhea, weakness, difficulty keeping balance, hyperexcitability, muscle spasm, seizures, coma and potentially death from an abnormal heart rhythm.
The following websites will give you’re the approximate amounts of chocolate that you will need to worry about based on the size of your dog and the type of chocolate. Consult this page to determine if a toxic amount has been ingested.
If a dog has ingested enough to be toxic according to the chart, you need to have the dog taken to the vet for supportive care. Your vet can administer charcoal to absorb the toxins and let it pass through your dog’s system with less absorbtion and monitor your dog’s vital signs until the danger has passed.
You also should be aware of the fact that the high fat content in chocolate can also trigger pancreatitis in some dogs and you may wish to take your dog into the vet anyway to avoid this condition developing. You can read about pancreatitis here:
Like I mentioned the small amount of chocolate involved here will probably have no effect on your dog.
Hope this helps.