Excellent to be concerned about heavy metal exposure. Cacao typically contains lead and cadmium; the reference dose (usually defined as: The amount of a chemical a person, including sensitive groups, can be exposed to on a daily basis over an extended period of time (usually a lifetime) without suffering deleterious effects.) of lead according to EPA is 3.5 ug/kg/day and cadmium is 1 ug/kg/day. The cadmium content of cacao nibs/paste is usually 0.5-1 mg/kg and 0.025 mg/kg lead. So cadmium is the one to be concerned about here; for a 100 kg person, the RfD would be 100 ug or 0.1 mg of cadmium per day. The FDA defines the toxicological reference value for cadmium as 0.2-0.3 ug/kg, or 20-30% of the EPA's RfD. 100 grams of cacao at the high side, 1mg/kg would contain 0.1 mg of cadmium. So a daily dose of 1-2 gram of cacao per kg of body weight should be within the RfD, and 0.2-0.6 grams cacao per kg body weight would be within the TRV. It should be noted that root vegetables such as potatoes and beets also tend to contain cadmium at a rate of about 1/10 per weight compared to cacao, but I would find myself just as likely to eat 500 grams of potato or beet vs 50 grams of cacao, which would net the same amount of dietary cadmium.
So definitely don't overdo it on the cacao and eat hundreds of grams per day, but IMO no reason to avoid it completely, relative to other vegetables that can accumulate cadmium. This is not medical or dietary advice.
