Peel and cook the yuccas in plenty of salted water for about 20 minutes or until they are tender. Drain and remove the inner root.
Mash the yucca so as to not leave any lumps. Add milk, the egg whites and parsley. Add more salt if necessary.
Take the amount desired to make a croquette, and with your finger press the dough in the middle, making a hole. Fill it with cheese and close it carefully.
Cover the balls of dough with flour and fry in hot oil. Serve immediately.