Prepare the guavas by trimming the edges and cutting them into pieces. Grind them in a mortar with water until you get a smooth paste.
2
Strain the ground guava mixture through a sieve to remove seeds and fibers, keeping only the liquid.
3
Pour the milk into a pot and add the evaporated milk. Heat gently on medium heat until warm but not boiling.
4
Pour the strained guava liquid into the warm milk, add the cinnamon stick, and bring to a boil for about 5 minutes.
5
Dissolve the cornstarch in cold water. Mix the reserved warm milk with the cornstarch solution, then slowly add this mixture to the pot while stirring constantly.
6
Continue stirring until the mixture comes to a boil again, ensuring there are no lumps. The atole is now ready to serve.