Boil the potatoes and mash them, add the flour, egg and salt. Then mix and pull out dough with a rolling pin. At this point cut the dough into squares of about 10 cm per side. Boil the spinach and, once drained, brown them in butter; by spoonfuls to have spinach on each square of dough and close by pressing the sides. The ravioli thus obtained are boiled for a few minutes in salted water and finally browned in melted butter. Season with plenty of grana cheese from Trentino.