This is how dogs die when they eat chocolate...same results — caffeine overload.
Chocolate contains theobromine. A naturally occurring stimulant found in the cocoa bean, theobromine increases urination and affects the central nervous system as well as heart muscle. While amounts vary by type of chocolate, it’s the theobromine that is poisonous to dogs.
From: http://www.petcaretips.net/chocolate-poisoning-dog.html