In a medium mixing bowl, combine the masarepa, sea salt, Cotija cheese, butter, and warm water. Knead until smooth, about 3-5 minutes. Let the dough rest for 10 minutes.

Divide the dough into 8 pieces. Roll into balls, flatten into disks, adding shredded mozzarella in the center, and seal the edges.

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

Transfer cooked arepas to a plate lined with paper towels. Serve warm to enjoy the gooey cheese inside.