Heat the oil in a heavy-based saucepan over a medium heat until the oil is sizzling. Using a spoon, sprinkle 4–5tablespoons of the parboiled rice across the bottom of the pan. Continue sprinkling the remaining rice, building it up into a dome shape. (Tipping it all in at once will compress the rice, and the end result will not be a light and fluffy dish.)
Use the handle of a wooden spoon to make three holes in the rice to the bottom of the pan. Melt the butter in a small saucepan over a low heat, then pour over the rice.
Wrap the saucepan lid in a clean dish towel and tie it into a tight knot at the handle, then cover the saucepan with the lid as tightly as you can so that any steam does not escape. (The dish towel will prevent the moisture from dripping into the rice and making it soggy.)
Reduce the heat to low and cook, covered, for another 20–40 minutes. If you cook for just 20 minutes, the rice will be light and fluffy and the tahdeeg will be golden, although quite loose; if you cook for the full 40 minutes, the rice will remain tender and fluffy but the tahdeeg will be firmer and darker, which is how it would be eaten in the Middle East. The choice is yours.
When the rice is cooked, place the saucepan in 5cm/2in cold water in the kitchen sink and leave for 1–2 minutes. This helps to shock the rice and loosen the tahdeeg.
Gently spoon the rice out (making sure not to disturb the tahdeeg), and sprinkle it lightly onto a dish, shaping it into a dome. Alternatively, gently tip the pan out onto the dish, allowing the rice to spill out into a mound.
Remove the tahdeeg by inverting the saucepan onto a plate, using a spatula to loosen it if necessary. Serve the tahdeeg separately on a plate or on top of the rice.