Loosen the membrane on the back of each rack of ribs by slipping a small sharp knife between the rib bone and the membrane. Using paper towels for grip, hold the membrane tightly and pull it off (check out the video above to see how I do this, or you can ask your butcher to do it for you). Discard the membrane.
In a small bowl, whisk together the brown sugar, salt, pepper, chili powder, granulated garlic, granulated onion and smoked paprika. Rub both sides of the ribs with the rub. If you have the time, cover the ribs and refrigerate them overnight (or for up to 2 days).
Preheat your smoker to 225˚F.
Place the rib racks on the smoker, meaty sides up, and cook for 3 hours.
Pull the ribs off the smoker (keep the smoker on with the lid closed). Arrange each rack on a large sheet of heavy duty aluminum foil. Brush the ribs with with ⅓ cup of the barbecue sauce (reserve the rest for later). Drizzle the apple cider and vinegar around the edges of the ribs. Dot with the top of the ribs with butter. Tightly wrap each rack in a double layer of foil to create a leak-proof packet. Return the ribs to the smoker and cook until tender when pierced with a knife, 2-3 hours longer.
Carefully unwrap the foil to expose the meat (be careful not to let any of the liquid in the bottom of the packets drip out—you will use it later!). Brush the racks with the remaining ⅓ cup barbecue sauce, and cook 15 minutes longer, or until the sauce turns sticky.
Transfer the rib racks to a cutting board (reserve the juices in the foil packets) and cut them into portions. Transfer to a platter and drizzle with the reserved juices. Serve with plenty of paper towels!