In a medium bowl, combine masarepa, sea salt, crumbled Cotija cheese, softened butter, and warm water. Stir until well combined, then knead for about 2-3 minutes until a smooth dough forms. Cover and let it rest for 10 minutes.

Divide the dough into 8 equal pieces, roll into balls, and flatten each into a circle about 5 inches in diameter. Place shredded mozzarella in the center, leaving a border, and seal with another circle of dough.

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

For air frying, preheat the air fryer to 350°F. Arrange arepas in a single layer, cook for about 5 minutes, flipping halfway until golden and crisp.

Remove the arepas and let them cool slightly. Serve warm, optionally adding a pat of butter on top.