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.
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Step 1: Procure the Chocolate
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.
Step 2: Melt and Cool the Chocolate
Put two ounces of the unsweetened baking chocolate with two cups of water in a small saucepan. Heat over medium-low, stirring until the chocolate is completely dissolved. Then leave it on the stove to cool. Historical sources suggest the Aztecs took their cacao cold.
Step 3: Froth the Chocolate
The traditional way to froth the chocolate would be to pour it from one bowl into another from shoulder-height. This can get messy! I recommend taking it outside or doing this over tile/linoleum -- something easy to clean up. As the foam builds, use a spoon to scoop it onto a plate or bowl for eating.
Or, if you'd like to get to the chocolate sooner and skip the pouring, grab an immersion blender and aerate the chocolate that way. An immersion blender makes a lot of foam very quickly.
Step 4: Experiment With Flavors!
The Aztecs flavored their chocolate with any number of things -- maize, achiote, vanilla, allspice, chili, honey, and a number of other flavorings not readily available on grocery store shelves.
I personally love chili in my chocolate, but when I'm making this with children, honey is easily the most popular addition.
For further reading on this topic, I highly recommend The True History of Chocolate by Sophie D. Coe and Michael D. Coe. They give a great amount of detail about the cacao plant, its history among the Maya, the trade of cacao up to the Aztecs, and the spread of chocolate into Spain and beyond.