Melt the butter in a large earthenware pot and add the potatoes. Leave them on a high heat until the butter is bubbling and then turn down to a gentle heat. Cook the potatoes for 30 minutes or so and then turn (they should be brown underneath).
Cook for a further 30 minutes or so until the potatoes are cooked through. Just before serving, add the cream and Parmesan cheese, gently amalgamating the ingredients.