How to Make It
In a large bowl, cover the rice with cold water. Using your hand, agitate the rice to release the starch, then carefully pour off the water. Rinse the rice 5 to 7 more times, until the water runs clear. Cover the rice with water, add 2 teaspoons of salt and let soak for 30 minutes, then drain.
Fill a 5-quart nonstick Dutch oven half full with water, bring to a boil and add 2 tablespoons of salt. Add the rice and boil over high heat until it is al dente and just starts to soften; this can take from 3 to 8 minutes, depending on your rice (do not overcook it). Let stand for 2 minutes, then drain well. Wipe out the pot.
In a medium bowl, mix the cilantro, parsley, chives and dill with the garlic. Put the oil in the bottom of the Dutch oven. In a small bowl, whisk the yogurt with the egg yolk and 2 tablespoons of the brewed saffron and drizzle over the oil in the pot. Gently scatter one-quarter of the rice into the pot (you want it to stay fluffy) and season with salt. Sprinkle with one-third of the herb mixture. Repeat the layering with the remaining rice and herb mixture, ending with a layer of rice.
Cover the pot and set it over moderately high heat until it starts to steam, about 5 minutes. Uncover the rice and drizzle with the remaining 2 tablespoons of brewed saffron and 1/4 cup of boiling water. Scatter the butter slices on top. Cover the pot with parchment paper, then cover tightly with the lid. Cook on a heat diffuser (if you have one) over moderately low heat until the rice is tender and very fragrant, about 45 minutes.
Discard the parchment. Carefully invert the rice onto a large platter. Using a spoon or spatula, crack the crust (tahdig) and spread the pieces apart. Drizzle the rice with a little brewed saffron and serve warm.