Prepare guava puree. Cut off ends of guavas and then cut into fourths. Add guavas and 1/2 cup water to a blender and process on low for 30 seconds. Strain mixture through a fine mesh sieve. Set aside.
Boil ingredients. Bring 4 cups water, cinnamon, rice, vanilla, and sugar to a boil over medium heat for 10 minutes.
Add thickeners. In the meantime, dissolve corn starch and corn flour in ½ cup of water and stir well until it’s completely smooth. Add corn slurry and milk to pot and cook over medium heat for an additional 10 minutes, stirring constantly. Do not walk away.
Simmer with guava puree. Reduce heat and simmer on low until slightly thickened and rice is tender. After about 15 minutes, add guava puree and simmer for an additional 5 to 10 minutes. Taste and adjust for sweetness. The entire process will take anywhere from 30 to 45 minutes, from start to finish. It should have a thick, drinkable consistency similar to a runny smoothie. I don’t like my atole super thick.
Serve and enjoy. Remove cinnamon sticks and pour into mugs. Serve with a sprinkle of cinnamon or a slice of guava, if desired. Be sure to scoop some of the rice into each mug.