Combine masa harina and 1 cup of water in a bowl. Set aside.
Place guavas in a medium saucepan with the sugar, cinnamon, and the remaining water.
Bring the water to a simmer and cook until the fruit is tender and falling apart.
Allow to lightly cool down, then transfer everything to a blender and blend until pureed.
Strain the pureed guavas into a medium pot. Discard solids.
Add the milk to the pot along with baking soda. Place the pot over medium heat.
Bring to a simmer and set the heat to medium-low.
Stir in the masa harina mixture while mixing with a spoon to prevent atole from forming lumps.
Set the heat to medium-low and cook for about 15 minutes or until the atole thickens to the point it coats the back of a spoon.
Adjust sweetness to taste, turn off the heat, and serve atole de guayaba in mugs.
Notes
Baking soda prevents your guayaba atole from curdling, don't skip this ingredient.
If you can't find fresh guavas, use canned. Those can be purchased online. Just make sure to use less sugar than the recipe calls for, as canned guavas are usually packed in sweet syrup.
You can substitute the masa harina with 4 tablespoons of corn starch (also known as Maizena) and mix with cold water before adding to the hot mixture.
To make it vegan or dairy-free you can replace whole milk with your favorite plant-based milk.