Chocolate toxicity calculator
They have to eat quite a bit - as in more than I eat in a serving. I ran the calculator for a 16# Jack Russell Terrier eating five dark Hershey's kisses (one ounce total; I generally have 2 or 3 kisses, or .4-.6 oz). The effect comes out "mild". I give my dogs a taste, perhaps 1/10 of one kiss, or .02 oz.
The bad elements also do not build up in the body, so toxicity is not cumulative.
Think I'm gonna go have some...