In a medium saucepan, combine ¾ cup of water, the strawberries, and half of the sugar. Bring to a gentle simmer over medium heat and cook for about 5–7 minutes, or until the strawberries are soft.
Transfer the cooked strawberries to a blender and blend until completely smooth. Strain the purée through a fine-mesh sieve to remove seeds. Set aside.
In a pot, add ¾ cup of water and the cinnamon stick. Heat over medium heat and let it gently simmer for 3–5 minutes to infuse the flavor.
Remove the cinnamon stick and pour the strained strawberry purée into the pot. Stir to combine.
Reduce the heat to low. Stir in the baking soda. The mixture will foam slightly — this is normal and helps neutralize the fruit’s acidity to prevent the milk from curdling.
In a separate bowl, dissolve the maizena (cornstarch) in cold water, whisking well until completely smooth and free of lumps. Slowly pour this mixture into the pot while stirring continuously.
Increase the heat to medium. Add the milk, evaporated milk, the remaining sugar, and vanilla extract to the pot. Stir well to combine.
Cook over medium heat, stirring constantly to prevent sticking or scorching. Bring the mixture to a gentle boil, reduce the heat and cook for an additional 3 minutes, stirring continuously.
The atole is ready when it thickens enough to lightly coat the back of a spoon. If you run your finger across the spoon, it should leave a clear line that holds its shape.