Heat clarified butter in a wok or frying pan over a medium heat. When hot, quickly add black mustard seeds and cumin seeds and then add the potatoes, stirring. Sprinkle in the turmeric and cook for about 15 minutes, shaking often, or until the potatoes are lightly browned and almost done. Just before the potatoes are done, sprinkle in the ground corriander, ground cumin, and cayenne and salt to taste. Drain off any excess butter in a colander.