The relatively high heat we use to brown the potato scales and the skin may burn the milk solids in regular butter. Ghee is pure butter with the milk solids removed, so it has a very clean flavor and does not contain the dairy that can burn. I love the flavor of it in this dish and the worry-free nature of using it for browning both the potatoes and the fish. For the butter sauce, regular butter is ideal as the dairy adds a luscious creamy quality to the sauce.