Cats should never consume chocolate. According the ASPCA, chocolate is toxic to cats due to the caffeine and theobromine, which are part of the methylxanthine drug class.
According to About.com, dark chocolate and unsweetened baker's chocolate contain more methylxanthines than milk chocolate. Unsweetened chocolate powder is the most toxic to cats because it has the highest levels of methylxanthines. White chocolate contains significantly lower levels of caffeine and theobromine and is relatively harmless to cats.
The ASPCA explains that when a cat consumes chocolate, it can exhibit symptoms that include vomiting, muscle tremors, irregular heartbeat, restlessness, seizures, increased thirst and increased body temperature. In severe cases, cats can die from ingesting chocolate.