Trim any large pieces of fat from the pork shoulder, but leave some for flavor.
In a small bowl, mix together salt, pepper, paprika, garlic powder, onion powder, and cayenne (if using).
Rub the spice mixture all over the pork.
Place the pork in the slow cooker. Add brown sugar, apple cider vinegar, barbecue sauce, and water.
Cover and cook on low for 10–12 hours or on high for 6–7 hours, until the meat is fall-apart tender.
Remove pork from the slow cooker, shred it using two forks, and return it to the juices.
Serve on buns with extra barbecue sauce, if desired.