Add the milk, Mexican chocolate, piloncillo, vanilla, salt and cinnamon stick to a medium saucepan. Heat over medium heat until the chocolate and sugar are dissolved. Stir frequently, about 3-5 minutes. Discard the cinnamon stick.
In a 4 cup measure, whisk together the warm water and masa harina until smooth. Carefully pour into hot choclate milk mixture, whisking to avoid lumps forming.
Bring to a simmer, reduce heat to low, and continue to cook for 25 minutes until thick, stirring frequently. Test - should coat back of spoon. Cook longer if you like it thicker, or thin with milk if you prefer it thinner.
Serve with Mexican pastries like Conchas or Marranitos.
Video
Notes
This atole can be made using all water (authentic, pre-Columbian era way) instead of milk. If using milk, whole milk is best. Almond milk or other nut milk will also work.
Two brands of Mexican chocolate are Abuelita and Ibarra chocolate, found in the Latin section of your grocery store or at your local Mercado. They are sold in 3 ounce discs. If using Hernan Mexican Chocolate, use 2 tablets.
If you can't find piloncillo, you can substitute 2 tablespoons of dark brown sugar.
You can substitute 1/4 tsp of ground cinnamon for one cinnamon stick.