Why is Cacao a stimulant?
Cacao and products made from cacao contain a number of bioactive compounds. As far as the subjective sensation of a "buzz", there are three related alkaloids present in cacao that may account for this effect. Theobromine is present in the highest concentration, but is probably less potent of a stimulant. It's not well researched. Theophylline has a number of effects, and is well studied. It has CNS stimulant effects, with a similar mechanism of action to caffeine, as a competitive antagonist at adenosine receptors. Caffeine is also present in cacao. The stimulant effects of caffeine are known generally, but if you'd like to read more about the pharmacology this is a good review.