Actually you need quite a bit of chocolate to kill a dog. My girlfriend's dog grabbed a Snickers bar out of a grocery bag one day; she freaked out and called an emergency vet and they said it would take the equivalent of ten snickers bars in one sitting to make the dog horribly ill and possibly kill him. If your dog gets a hold of a brownie or a candy bar just give it a slice or two of bread and the bread will keep it from getting too sick.
The reason you have to worry about dogs is because they're gluttons. They'll eat and eat and eat. Put ten pounds of chocolate where a dog can get to it and he'll eat himself to death.
Cats, on the other hand, won't. It's not because they're smart (though they undoubtedly are and my kitties are adorable proof of this), but because it doesn't taste good to them. Dogs aren't obligate carnivores. Cats are. An obligate carnivore is one that can
only survive on a diet of meat. The internal biology of the cat reflects this; it's intestines are much, much shorter than non-obligate-carnivorous animals. The digestion of proteins takes place primarily in the stomach and their absorption by the body happens almost immediately afterward in the intestines. Most of the rest of the length of the intestines is devoted to the breakdown of carbohydrates by the parasites that rent space in our intestines. Cats, not having those intestines, can't benefit from non-protein foodstuffs. Therefor, they've evolved not to even
taste other foodstuffs.
We taste "salt", "sweet", "sour", "bitter", and "umami". Cats don't taste "sweet", and are rather lean for "sour" and "bitter". They're loaded with tastebuds for "umami". "Umami" is, I believe, Japanese for "satisfaction" or "satiation" and is the taste we associate with meat and protein. Cats are loaded for bear when it comes to savoring the delicious taste of flesh. And good thing, too, because it's the only thing they can eat.
Since they don't taste sweet, however, most of the rest of the things we eat are wholly unappetizing to them. They won't eat chocolate because all they can taste is its bitterness.
Dogs, with their longer intestines, can eat non-protein foods and benefit from them. They can also taste "sweet". And they're also terrible gluttons. So they'll devour chocolate until the chemicals it contains kill them.