Steps
1. Clean, wash, and soak rice for 30 mn. Cook in a pan with 4 cups water. Boil on high heat for 5 mn. Reduce heat, stir rice, and leave on low heat again for 5 mn, then drain in a sieve.
2. Mix saffron and warm water; set aside. Peel potatoes and cut into thick round slices.
3. Pour saffron in a pan, add 2 tbsp melted butter and potatoes, and mix very well.
4. Spread sliced potatoes in the bottom of the pan and spoon all rice over.
5. Sprinkle with remaining butter. Cover pan with aluminum foil and leave on very low fire until done.
6. Transfer rice to a platter so that sliced potatoes are on top of rice.

Source
International Rice Institute (IRRI) & International Women's Organization (SUHAY), VIRMANI, Inderjeet (ED)(1991), Home Chefs of the World - Rice and Rice-based Recipes, ISBN 971-22-0023-X, Manila, Philippines