This article didn't go into it, but here's a little background info from how some of these heavy metals end up in chocolate (at least):
The researchers found that cacao plants take up cadmium from the soil, with the metal accumulating in cacao beans as the tree grows. That’s similar to how heavy metals contaminate some other foods.
But lead seems to get into cacao after beans are harvested. The researchers found that the metal was typically on the outer shell of the cocoa bean, not in the bean itself. Moreover, lead levels were low soon after beans were picked and removed from pods but increased as beans dried in the sun for days. During that time, lead-filled dust and dirt accumulated on the beans. “We collected beans on the ground that were heavily loaded with lead on the outer shell,” DiBartolomeis says.
The link from my quote includes a breakdown of the different types of bars and how much was found in each, so I would compare the kind of chocolate you usually eat, the safest choices were:
Edit 2: Note this particular study was done on dark chocolate bars. Milk chocolate bars would be more diluted.
Mast - Organic Dark Chocolate\80% Cocoa
Taza Chocolate - Organic Deliciously Dark Chocolate\70% Cacao
Ghirardelli - Intense Dark Chocolate\86% Cacao
Ghirardelli - Intense Dark Chocolate Twilight Delight\72% Cacao
Valrhona - Abinao Dark Chocolate\85% Cacao
Even those contained small levels of both, so eating an entire bar all at once is probably not a good idea.
Also there are other foods that can have it:
It can be found in many other foods—such as sweet potatoes, spinach, and carrots—and small amounts from multiple sources can add up to dangerous levels. That’s why it’s important to limit exposure when you can.
To my knowledge, that is only lead acetate.
If it was lead acetate that would be a obvious additive, however the article talks about how the metals are accumulated from the cocoa plants themselves through the soil.
Lead supposedly does not have an unpleasant taste, unlike other poisons and heavy metals. Combine it with some high-fructose corn syrup and you have a delicious path to heavy metal poisoning!