Cook the plantains in a pot with water for about 30 minutes. Let them cool for about 5 minutes, then peel the cooked ripe plantains.
Mash the plantains and add the masarepa, milk, melted butter and salt to your taste. Let mixture stand for five minutes.
Make small balls with the mixture. Shape the dough into thick cakes.
Add the butter or oil spray 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 they are golden brown. Serve them warm with butter and cheese or as a side dish.