01 -
Pat the ribs dry. Remove the membrane from the bone side, then cut each rack in half so the pieces fit in the slow cooker.
02 -
Mix the brown sugar, smoked paprika, garlic powder, onion powder, salt, chili powder, black pepper, and cayenne. Press the rub over both sides of the ribs.
03 -
In a 6-quart slow cooker, stir together the barbecue sauce, apple cider vinegar, Worcestershire sauce, mustard, and water if needed.
04 -
Add the ribs to the slow cooker, standing them around the sides or layering them gently. Spoon some sauce over the top. Cover and cook on low for 6-7 hours, or on high for 3 1/2-4 hours, until tender.
05 -
Heat the broiler. Transfer the ribs to a foil-lined baking sheet, meaty side up. Brush with sauce from the slow cooker or fresh barbecue sauce and broil for 3-5 minutes until bubbling and caramelized.
06 -
Let the ribs rest for 5 minutes, then cut between the bones. Serve with extra warm barbecue sauce.