Add the cream (or coconut milk), water, maca, cocoa, MCT oil powder, cinnamon, salt, and sweetener to a small saucepan over medium-high heat. Whisk until the powders are incorporated into the liquids, about a minute.
Continue whisking occasionally as the cream heats up. Taste it about halfway through and if it is sweet enough to your liking, leave it as-is. If it needs more, add it now and taste again. Repeat until you get it where you want it.
Before it comes to a boil, spoon out a little of the cocoa and taste it to be sure it's the right temperature for you (you don't want to burn your tongue on it), remove from heat. Add the vanilla.
Put an immersion blender directly into the pan, placed on the bottom, and turn onto the highest setting, moving it around the bottom in a circular motion. Continue this for about a minute to emulsify the cocoa and add foam to the top.
Alternately, pour the prepared cocoa into a blender and run on HIGH for 30-45 seconds. This will give you a lot of thick foam on top (which I really like).
Pour directly into a large mug (not a regular small one or it might not all fit) and top with an additional sprinkle of ground cinnamon, if desired. Settling may occur if you don't drink it right away, so stir it up after a little while.