Best-Ever Barbecue Ribs

Preheat oven to 300°. Combine 2½ Tbsp. Diamond Crystal or 4½ tsp. Morton kosher salt, 1 Tbsp. dry mustard, 1 Tbsp. paprika, ½ tsp. cayenne pepper, and ½ tsp. freshly ground black pepper in a small bowl. Place each rack of 8 lb. baby back ribs or St. Louis–style spareribs on a separate double layer of aluminum foil; sprinkle spice rub all over ribs. Wrap racks individually in foil and divide between 2 baking sheets.

Bake ribs until very tender but not falling apart, about 2 hours for baby back ribs or 3 hours for spareribs. Carefully unwrap ribs; pour off any juices from foil into a heatproof measuring cup and reserve for another use. Let ribs cool completely. Do Ahead: Ribs can be baked 3 days ahead (the flavor will be more developed, and the cold ribs will hold together better on the gril). Rewrap cooked ribs in foil and chill.

Build a medium-hot fire in a charcoal grill, or heat a gas grill to high. Grill ribs, basting with up to 3 cups 15-minute BBQ Sauce or store-bought barbecue sauce, turning frequently, until lacquered and charred in places and heated through, 7–10 minutes. Transfer ribs to a cutting board and let cool slightly. Cut between ribs to separate. Transfer to a platter and serve with any remaining barbecue sauce.