Mix together cornstarch with one cup of milk until well combined.
Place the remaining milk into a large pot with the cinnamon stick.
Heat the milk over medium heat while stirring constantly for 5 minutes.
Add the cajeta sauce and mix for 1-2 minutes to combine everything.
Slowly and while mixing, add the cornstarch mixture to the pot. Then add also the vanilla extract.
Keep cooking the atole for 10 minutes over low heat and mixing constantly until it will have a smooth and slightly thick texture.
Serve hot in mugs and enjoy.
Notes
Add the cornstarch mixture slowly and while whisking, to prevent atole from forming lumps.
Cook the atole mixture always over low-medium heat to prevent it from sticking to the pot and burning, which will add an unpleasant flavor to the drink.
Never leave the pot unattended or the milk mixture will boil and spill out.
Substitute 2 cups of milk with 1 can of evaporated milk to make it creamier.
You can also use 4 tablespoons of masa harina as a substitute for cornstarch, it will give this atole a slightly grainy texture, typical of this beverage.
To prevent skin from forming on top of the atole while cooling down, give it a quick mix from time to time.