Add the corn and 2 cups of milk to a large blender. Blend until smooth.
In a medium pot or saucepan, bring the water to a simmer over medium-low heat. Add the cinnamon stick and simmer for 3 minutes. Add the piloncillo and stir until fully dissolved.
Strain the corn mixture into the pot using a fine mesh strainer. Stir in 1 cup of the remaining milk and the vanilla extract. Continue to gently simmer the atole mixture for 5 minutes, stirring continuously.
Make a cornstarch slurry by whisking together the cornstarch with the remaining 1 cup of milk until fully combined. Slowly pour it into the pot while whisking.
Gently simmer the atole for 10-15 minutes to allow the mixture to thicken, until it coats the back of a spoon.
Remove the cinnamon stick and serve immediately in mugs with a dusting of ground cinnamon.
Notes
Corn: You can substitute 2 (15-ounce) cans of drained whole-kernel corn instead of fresh corn or 3 cups of frozen corn that’s been thawed.
Piloncillo: You can substitute 1 cup of brown sugar instead of the piloncillo.
Milk: Using whole milk adds a creamy texture and flavor, but you can make this dairy-free by using almond or oat milk.
Cinnamon: I recommend using Ceylon cinnamon as that is what's used in Mexican cuisine, but you can use whichever cinnamon sticks you can find.