Soak the rice in water and a hefty pinch of salt for 1 hour.
After 1 hour bring a large pot of water to a boil then add 1/4 cup of salt.
Drain the soaked rice then add it to the boiling salt water and parboil for 5 minutes.
After 5 minutes, the rice should puff up slightly and be soft on the outside but still a little hard on the inside.
Strain into a fine mesh colander and rinse under cold water to stop the cooking. Taste the rice- if it is too salty, rinse it some more.
Strain the rice well.
Grind up the saffron threads with your fingers or a mortar and pestle. Dissolve with 2 tbsp of hot water. Let it infuse for 10-15 minutes.
Mix 1 1/2 to 2 cups of the rice with yogurt, oil, saffron water, and 1 tbsp melted butter.
Melt the remaining 1 tbsp butter in a non stick pan or dutch oven. Use a little more butter if you're using a pan with a larger diameter, like 10 + inches.
Add the saffron & yogurt rice, spreading evenly but do not pack it down too tighlty.
Then add the remaining parboiled rice on top. Poke 6 to 8 holes into the rice using the back of a wooden spoon or spatula. This allows steam to escape so the bottom can get crispy.
Wrap the lid with a kitchen towel, then cover and turn up the heat to medium for 7 minutes. Then turn the heat down to low for 40 minutes.
Turn the pot by 1/4 turn every 10 minutes so the bottom cooks evenly.
When finished, you can use a flexible spatula to carve around the edges to help it come out cleaner. Press down and let the spatula bend underneath the tahdig to release it from the pan.Remove the lid and place a plate upside down over the top of the pot. Carefully grasp both sides and flip it over, then lift the pot upwards to release the tahdig onto the plate.
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