Using a mortar and pestle, coarsely crush the garlic, coriander, peppercorns, salt and optional saltpetre. Rub the mixture into the brisket, and then place the meat in a ceramic container just large enough to accommodate it. Cover with a tight-fitting lid or with foil. Refrigerate for 24 hours.
In a small bowl, mix together the oil, 3 tablespoons of brown sugar, the vinegar and allspice. Pour the mixture over the meat. Marinate in the refrigerator for another 24 hours, turning once.
Place the meat on a steamer tray set into a steamer with just enough water to steam the meat for 2-1/2 hours. Pour the marinade over the meat. Some or most of the marinade will drip down into the water below. Bring the water to a boil, then reduce the heat so that the water boils constantly to create steam but doesn't boil up and over. Cover the pot tightly with a lid or with foil. Steam for 2-1/2 hours or until the meat is tender but not falling apart. Turn off heat. Remove the steamer tray with the meat. Clean the steamer pot. Line the bottom and about an inch up the sides with foil.
Place 1 cup brown sugar on the foil and return the steamer tray with the meat to the steamer, cover, and adjust the heat to medium. When you start to smell the brown sugar smoking, reduce the heat so that the sugar continues to smoke without burning. Smoke the meat for 15 minutes, then turn off the heat and leave the steamer on the burner until cool.
Slice the meat thin, and pile high on rye bread!