In a mixing bowl, combine the masarepa, sea salt, Cotija cheese, melted butter, and warm water. Stir until a dough forms, then knead for 2-3 minutes until smooth and pliable. Cover the bowl with a kitchen towel and let the dough rest for 10 minutes.

After resting, take a small piece of dough and flatten it into a disc. If the edges crack, add warm water one tablespoon at a time, kneading gently after each addition until smooth.

Divide the dough into eight equal pieces and roll each into a ball. Flatten each into a 5-inch circle, placing a generous portion of mozzarella cheese in the center of half of the discs. Cover with the other half, sealing the edges.

Heat a skillet over medium-low heat and add oil or butter. Cook the arepas for 4-5 minutes on each side until golden brown and crispy.

Transfer the Arepas con Queso to a paper towel-lined plate to absorb excess oil. Serve warm and enjoy!