In a mixing bowl, combine masa harina and salt. Mix thoroughly to ensure the salt is evenly distributed throughout the masa harina.
Add the warm water gradually while mixing with a spoon until the mixture thickens. You may need to adjust the water slightly depending on the humidity.
Start kneading with your hands until a smooth dough forms. This should take about five minutes. The dough should be soft and pliable; if it’s too sticky, add a bit more masa harina.
Let it rest for about five minutes. This resting period allows the dough to develop its texture.
Divide the dough into fourteen equal balls. You can use a kitchen scale for precision if desired.
With your hands, form a pocket in the middle of each ball. This is where the filling will go!
Place one teaspoon of refried beans and one tablespoon of shredded mozzarella cheese in the center of the pocket. Be careful not to overfill them!
Using the edges of the dough, cover the filling and form the ball once again. Ensure the filling is completely sealed inside.
Flatten each filled ball into a thick disk shape, about half an inch thick. This can be done with your hands or by using a flat surface.
Add oil to a non-stick pan and place it over medium heat. Once hot, add the Pupusa to the pan.
Cook for three minutes on one side until it starts browning. Flip and repeat on the other side until golden brown.
Once cooked, remove the Pupusas from the pan and let them cool slightly before serving.