Wash and rinse the rice with cold water until the water runs clear. Soak the rice in 6 cups of cold water with 6 tablespoons of salt and set aside for a couple of hours. Drain but don’t rinse the rice. Make sure that you don’t use your hand or a utensil during this process. Basmati rice is very delicate and can easily break.
In a large non-stick pot, add 6–8 cups of water and 2 tablespoons of vegetable oil, and bring to a boil. Add the drained rice to the boiling water and cook for about 7 minutes, until the rice is parboiled or al dente—the grain needs to be soft from the outside but firm in the centre. Drain and rinse the rice in cold water.
Wash and dry the pot. On medium–high heat, melt ¼ cups of butter, add little bit of vegetable oil. In a small bowl, add one cup of cooked rice, yogurt and a pinch of ground saffron; mix together and spread the mixture on the base of the pot. Gently add the rest of the parboiled rice, and make three holes in the layer of the rice with the handle of a spatula or wooden spoon.
In a small cup, mix half a cup of hot water with 3 tablespoons of butter and a pinch of saffron, and sprinkle the mixture over the rice. Once the rice starts to steam, wrap the pot with a towel and reduce the heat to low. (This allows the rice to absorb the excess liquid and become fluffy.) Cook for about 45–50 minutes.
Spoon the rice on to a platter. Reserve the crispy rice crust (tahdig) on the side and serve. The tahdig is usually the most fought-after element of this traditional Persian dish.