In a mixing bowl, combine masarepa, warm water, and salt.
Mix with your hands until a smooth dough forms. It should be moist but not sticky.
If needed, add additional water one tablespoon at a time or a little extra masarepa to adjust consistency.
Let the dough rest for 5 minutes.
Divide dough into 4 equal portions.
Roll each portion into a ball and flatten into a disc about ½-inch thick and 3–4 inches wide.
Smooth any cracks around the edges with damp fingers.
Heat a skillet or griddle over medium heat and melt the butter.
Cook arepas for 5–6 minutes per side until golden brown and crisp.
Transfer to a plate and cool slightly.
Carefully slice a pocket into each arepa without cutting all the way through.
Fill each arepa with shredded chicken and mozzarella cheese.
Return to the skillet and cook 2–3 minutes, covered, until the cheese melts and the filling is heated through.
Serve hot with avocado, sour cream, or hot sauce.