Wash the rice under running water several times then leave it to steep in a bowl of water for 3 hours.
Bring a large saucepan of salted water to the boil and add the drained rice in a stream. Cook for 5 minutes, or until tender on the outside and still firm inside; drain.
Melt the butter in a heavy-based saucepan and add the rice. Stir well then cover the pan tightly. Cook over a medium heat for 15 minutes. Lower the heat and continue cooking over a very, very low heat for a further 20 minutes.
Dissolve a pinch of saffron threads in 2 tablespoons of hot water. Take a bowlful of rice from the pan and sprinkle it with the saffron-flavoured water.
Place the remaining rice in a large serving dish and top it with a little dome of the saffron rice.
Scrape up the rice crusted around the bottom of the pan, and serve it separately on a small plate.