In a small bowl, mix together the brown sugar, smoked paprika, salt, black pepper, garlic powder, onion powder, and cayenne pepper (if using) to make the dry rub.
Remove the membrane from the back of the ribs if present. Pat the ribs dry with paper towels.
Generously coat both sides of the ribs with the dry rub, pressing it into the meat.
Place the ribs in the slow cooker, curling them around the inside if needed to fit.
Cover and cook on low for 6 hours, or until the ribs are tender.
Carefully remove the ribs from the slow cooker (they will be very tender).
Brush the ribs with barbecue sauce. For extra caramelization, place under a broiler for 3–5 minutes or grill briefly, if desired.
Slice and serve with additional barbecue sauce if desired.