It is the theobromine, a stimulant, found in the cocoa that is harmful and can cause irregular heart beating, speeded up heart beat and even heart failure.
A dog with theobromine poisoning will probably start to look generally ill with symptoms like vomiting etc..but I've read of cases where there was no warning and dogs just collapsed and died suddenly after appearing fine. This happened to two bulldogs who were fed a slice of spongecake with chocolate on top as a treat about half an hour before. They belonged to a breeder who had about 7 dogs in all and they should've known better than to be feeding rubbish like that to their dogs. Obviously the amount they consumed was far lower than what would be expected to make a dog their size ill and even more strange was that one of them used to get fed a bar of choc every evening all his life as a treat and it never did anything and then he died from this slice of cake. The other was a bitch. Both died about 7 minutes apart whilst out on a walk. The other dogs were unaffected.

Obviously, dark choc has higher amounts of theobromine in it than milk choc or white choc which has the least. Cocoa powder is the worst.
Obviously, you'll have learnt not to leave any chocolate anywhere where the dog can get to it now.

If your dog starts to look unwell, be sure to get to a vets straight away because theobromine poisoning can be severe.

Humans are ok with theobromine because our bodies excrete it before it has a chance to do harm to us. Dogs and cats can't but then cats aren't usually into sweet stuff because they don't really have tastebuds that taste for sugar and sweet things...being obligatory carnivores and all.

Here is a link with info about general amounts needed to do harm etc..although, as it states, dogs can die from eating smaller amounts.

Chocolate poisoning

Anyone who feeds their dogs choc bars each day needs their heads reading. Even without the theobromine danger, they're full of sugar, fattening and not a good substitute for proper food.