Ground cinnamon, for garnish - Ground nutmeg works too.
Whisk 1/2 cup whole milk with the cornstarch until you get a smooth mixture. Set it aside.
Add the evaporated milk, hot chocolate, remaining milk, and cornstarch slurry to a large pot over medium-low heat. Whisk to combine.
Bring the mixture to a boil over medium-high heat. Immediately reduce the heat to low. Let the atole simmer for 15-20 minutes or until thickened to taste. Stir constantly. Scrape the bottom and sides of the pot so that nothing sticks.
Remove the pot from the heat. Taste test the atole and add dark brown sugar to taste if needed. Stir well. Pour it into a mug and sprinkle with ground cinnamon to taste.
Storage: Once fully cooled, refrigerate it in an airtight container for up to 4 days. It will thicken slightly so you'll have to add 2-5 tablespoons of milk to get it to the right consistency again.