Place warm water and salt in a large bowl. Stir until the salt dissolves. Gradually, add corn flour, stirring with a spoon to combine. Incorporate melted butter and mix well.
Add the shredded mozzarella and incorporate it with your hands. Let it sit for 5 minutes. The dough should be soft and manageable, not too sticky.
Make 6 balls from the dough, then gently flatten them into discs about 0.5-inch thick. Place a cube of mozzarella in the center, and using your hands, arrange the dough around it, making sure it stays in the center of the arepa.
Preheat an iron cast skillet over medium heat. Cook each arepa on both sides until browned, about 4-5 minutes per side. After browning them in the skillet, finish cooking them in a preheated oven at 350°F for 8-10 minutes.
Enjoy the arepas hot, either on their own or with extra cheese, butter, or any other topping you like.