Step 1Place the couscous in a sieve fine enough to hold all the grains. Place the sieve under cold running water and wet the couscous thoroughly. Spread it out in a thick layer on a large rimmed baking sheet (ideally 16 to 18 inches wide) or in a huge wooden salad bowl. Let it stand for 10 to 15 minutes to absorb the moisture.
Step 2Evenly sprinkle about 2 tablespoons of oil over the couscous and work the oil into the grains with your fingers. Take a small handful of grains and rub them between your thumb and two middle fingers to get rid of all the little lumps. As you work, lift the grains up, letting them drop back into the sheet. Alternatively, work over another large bowl to keep track of what's been worked over. Repeat with all the couscous.
Step 3Oil the perforated top of a couscousiere or a large wide steamer. In the bottom part, bring 4 to 5 inches of water to a rolling boil. If there is a lot of steam escaping through the seam between the two parts, seal it with foil. When you see a good deal of steam rising up, transfer the couscous to the top part and steam uncovered for 15 to 18 minutes.
Step 4Return the couscous to the baking sheet and spread it out again, breaking the lumps with a fork, and let cool until just manageable. In a measuring cup, combine 2 cups water and salt, and sprinkle evenly and gradually over the couscous, working the water in with your fingers. Spread it out again and let stand for another 10 to 15 minutes.
Step 5Oil your hands and repeat the procedure in Step 4. Steam again as in previous step for about 15 minutes. By now, the grains will be fluffy and separate and slightly al dente. If not planning to steam couscous for a third time (instructions follow), steam it now for about 20 to 30 minutes until tender.
Step 6If steaming for a third time, return the couscous to the baking sheet, work in 1 more cup of water, spread it out again and let stand until ready for the final steaming. Before steaming, rub it again to remove lumps and steam one final time, 12 to 15 minutes.
Step 7Turn off the heat and leave the couscous in the steamer. Carefully stir in the butter, using a fork to break up any lumps that might have formed.