Remove the thin membrane from the back of each rib rack. Slide a butter knife under the membrane at one end, then grip it with a paper towel and pull it off. This is the single most important step for tenderness.

In a small bowl, mix the brown sugar, smoked paprika, garlic powder, onion powder, salt, black pepper, and cayenne if using. Rub this mixture all over both racks of ribs, coating every surface. Let them sit at room temperature for 15 minutes.

Place the ribs in the slow cooker, standing them up along the sides if needed, or cutting each rack in half to fit. Do not add any liquid. The ribs will release their own juices.

Cook on LOW for 7-8 hours, or on HIGH for 4-5 hours. The meat should be tender but not falling completely off the bone when you lift the rack with tongs. Overcooking makes them mushy, so check at the 7-hour mark on low.

About 20 minutes before the ribs are done, preheat your oven broiler to high. Line a baking sheet with aluminum foil.

Carefully transfer the ribs from the slow cooker to the prepared baking sheet, meaty side up. Discard the cooking liquid.

In a small bowl, mix the barbecue sauce with the apple cider vinegar. Brush a generous layer over the tops of the ribs.

Broil for 3-5 minutes, watching closely, until the sauce is bubbly and beginning to char in spots. Remove from the oven, let rest for 5 minutes, then slice between the bones and serve with extra sauce on the side.