Chocolate contains a substance called theobromine which is highly toxic to dogs and cats. Animals that metabolize theobromine more slowly, such as dogs, can succumb to theobromine poisoning from as little as 50 grams (1.8 oz) of chocolate for a smaller dog and 400 grams (14.4 oz) for an average-sized dog.

If it was carob chocolate, it'd be fine (as it contains no theobromine or caffeine, both of which are toxic to dogs and cats).