In a medium bowl mix the masarepa, water, salt and butter. Knead with your hands for about 3 minutes moistening your hands with water as you work.
Form 6 small balls with the dough. Place each ball between 2 plastic bags or parchment paper and with a flat pot cover flatten to about ⅓ inch. The arepas should be about 5” in diameter.
Add the butter to a nonstick pan over medium heat. Place the arepas in the pan, and cook about 3 minutes on each side, until a crust forms or until they are golden brown.
Split the arepas using a knife as you would do with an English muffin and stuff wit 2 slices of mozzarella cheese.
Place the stuffed arepas back on the skillet over medium high heat and cook 2 minutes on each side or until the cheese is melted and serve immediately.