If in doubt, always call your vet or the Animal Poison Line immediately.
Theobromine and caffeine stimulate the heart and nervous system. Dogs metabolise these compounds far more slowly than humans, causing a toxic build-up. Dark and baking chocolate have the highest levels : milk chocolate is less concentrated but still dangerous.
Contact your vet or emergency animal hospital immediately. If ingested within 2 hours, your vet may induce vomiting. Bring the chocolate packaging so the vet can calculate the theobromine dose.