Carefully wash sea-kale to remove grit, remove any black parts from the roots and tie up the shoots in small bundles. Cook in boiling salted water for twenty minutes, drain and keep hot. Mix on the fire the butter and flour, moisten with half a cup of water in which the kale was cooked, bring to a boil and mix in the grated Parmesan cheese. Take from the fire and add the beaten yolk. Arrange the kale on a hot dish, pour the sauce over and serve immediately.