Heat oven to 300°F with a rack in the center position.
In a small bowl, combine the salt, chili powder, paprika, garlic powder, onion powder, pepper, and cayenne.
If the ribs still have the thin clear membrane covering the back of the rack, you will need to remove it. Using a paring knife, start at one end of the ribs and carefully slide it under the membrane until you have separated enough of the membrane to hold onto. Use a paper towel to hold your grip on the membrane. Using a spoon or paring knife, slowly pull back on the membrane until it separates from the rack. Discard.
Rub the seasoning all over the ribs. Place the ribs, meat-side up, on a large rimmed sheet pan. If necessary, trim a few ribs off the rack and arrange them to fit on the pan.
Cover the baking sheet tightly with aluminum foil, and bake for 2½ to 3 hours, or until the meat easily falls off the bone.
Remove the ribs from the oven. Set aside the foil, then brush both sides of the cooked ribs with ½ cup of barbecue sauce.
Turn the broiler to HIGH. Return the ribs to the oven and broil for 2 to 4 minutes, just until the barbecue sauce begins to caramelize. Remove from the oven, lay the foil over the ribs and let them rest for 10 minutes.
Serve warm, with the remaining bbq sauce alongside.