In a medium bowl combine the masarepa, salt, crumbled cheese, butter and warm water.
Knead the mixture, until a smooth dough is formed. The dough should not be sticky but have the consistency of homemade play-dough. Cover the bowl with a kitchen towel and allow it to rest for 10 minutes.
Check the consistency of the dough by flattening a small amount of dough between the palm of your hands. If the edges crack, the dough is too dry. Add additional water, one tablespoon at a time until the dough is pliable.
With lightly wet hands, divide the dough into 8 even pieces. Roll each piece into a ball then, pat each dough ball into a circle that is approximately 5-inches in diameter and about ¼” thick.
Working with one circle at a time, add a couple of tablespoons of shredded mozzarella cheese in the center leaving about a 1/2-inch border. Place a second dough circle on top and seal the edges. Press the dough lightly between the palm of your hands smoothing the sealed edges to form a disk of about 1/2 inch thick. Repeat with the remaining dough and cheese.
To cook the arepas on the stove top: Heat the oil in a large skillet over medium-low heat. If using butter, heat the skillet and then add the butter, spreading it around to coat the skillet.
To cook the arepas in an electric griddle: Heat the griddle to 325ºF. Add the butter and spread it around to coat the griddle’s surface.
Place the arepas on the skillet or griddle and cook, moving them around occasionally for about 4-5 minutes or until golden brown. Flip and cook for about 4-5 minutes or until golden brown and crispy. Remove from the skillet and serve warm, with additional butter on top (optional).