Theobromine, the caffeine analogue which gives chocolate its kick, is highly toxic to dogs and cats, and is the reason why you shouldn't let those particular four-feets near your chocolate. Their livers are unable to break it down, and it can quickly build up to a lethal level in the bloodstream.
Theobromine is only present in chocolate liquor, however. White chocolate, being mostly cocoa butter, doesn't have it, so if your furry friend must have chocolate, give white chocolate. I've seen dog biscuits dipped in white chocolate at a candy store out on the Eastern Shore, along with a note explaining why white chocolate is safe for dogs.