Stir the chili and cumin together with 1 tablespoon salt and 1/2 teaspoon pepper in a small bowl. Sprinkle the mixture all over the ribs, then put the ribs bone-side down in a large roasting pan and cover tightly with foil. Bake until the meat is very tender but not quite falling off the bone, about 2 1/2 hours.
For the sauce: Meanwhile, melt the butter in a medium saucepan over medium-high heat. Add the peppercorns, garlic and shallot and cook, stirring, until the garlic starts to turn golden, about 2 minutes. Add the sugar, preserves, honey, ketchup, vinegar, soy sauce and Worcestershire and bring to a boil. Reduce the heat and simmer until syrupy, about 5 minutes.
Finish the ribs: Preheat the broiler to high. Remove the foil and transfer the racks to a cutting board to cool slightly. Drain any fat from the roasting pan. Cut the racks into individual ribs and place back in the roasting pan. Drizzle the ribs with the sauce.
Broil the ribs a few inches from the broiler, tossing occasionally with tongs, until bubbling and charred in spots, about 6 minutes. Transfer the ribs to a platter and sprinkle with the scallions.