Heat oven to 375 degrees.
Trim off any loose bits of meat and excess fat, leaving about 1/4 inch of fat. Put lamb in a roasting pan skin side up, cover with aluminum foil, and roast for 1 1/2 hours.
Remove from oven, uncover, and rub generously with butter. Cover with foil and return lamb to oven. Repeat 3 more times, once every 15 minutes.
After 3 1/2 hours, remove foil, raise heat to 425 degrees and cook 10 more minutes, or until the skin side is gold and crusty.
Remove from oven, place shoulder on a large platter, and let rest for 5 minutes. Sprinkle with salt and ground cumin. Carve and serve with salt and cumin in small dishes on the side.
Recipe courtesy of Lofti's restaurant, New York, NY