In this instructable, I'll teach you how to make authentic Aztec chocolate. While the Aztecs didn't leave us a step-by-step recipe for what they drank, we do have historical descriptions like this:
"And after having mixed it [ground cacao] very well, they change it from one basin to another, so that a foam is raised which they put in a vessel made for the purpose. And when they wish to drink it, they mix it with certain small spoons of gold or silver or wood, and drink it, and drinking it one must open one's mouth, because being foam one must give it room to subside, and go down bit by bit."
(Quote from Anonymous Conquistador, 1556, translated by Sophie D. Coe and Michael D. Coe in The True History of Chocolate, 3rd Edition, Thames & Hudson 2013)
After a good deal of experimenting, I've recreated a method for getting great chocolate foam that the Aztecs might have recognized.
From a culinary standpoint, fat makes bubbles -- that's why whipping cream gives us delicious peaks and whisking skim milk is an exercise in futility. I don't have good enough grinding equipment to turn my own cacao nibs into chocolate liquor -- a smoothly-ground substance of roughly half cacao solids and half cacao butter.
Thankfully, most unsweetened baking chocolate is nothing but chocolate liquor. Some brands are full of other things though, so check the label. It should say "chocolate" -- nothing more, nothing less.