In a small bowl, mix together brown sugar, smoked paprika, garlic powder, onion powder, ground black pepper, and cayenne pepper.
Rub the mixture evenly over both sides of the ribs.
Place the ribs in the slow cooker, meat side up.
Pour the barbecue sauce over the ribs, ensuring they are well coated.
Cover and cook on low for 8 hours or until the ribs are tender and cooked through.
Optional: For a caramelized exterior, place ribs on a baking sheet and broil in the oven for 3-5 minutes after slow cooking.