Add masa harina, water, melted butter & salt to a bowl. Mix until the masa has absorbed the liquids & forms a cohesive mass when squeezed with your hand. Cover with plastic wrap & set aside while making the filling.
Pour the hot filling onto the crust, top with remaining cheddar cheese and bake for 25-30 minutes or until cheese is melted, bubbling & slightly golden. Let stand for 15 min. Serve with sour cream, cilantro & avocado.