Prep the Ribs: Remove the membrane from the back of the ribs for better seasoning penetration.
Coat the ribs with mustard, then apply a dry rub made of brown sugar, paprika, garlic powder, onion powder, chili powder, black pepper, salt, and cayenne pepper. Let rest for 30 minutes.
Preheat the Smoker: Set the Traeger smoker to 225°F and preheat for 15 minutes.
Smoke the Ribs: Place the ribs on the grill, bone side down, and smoke for 3 hours, spritzing with apple cider vinegar every hour.
Wrap the Ribs: Wrap the ribs in foil with brown sugar and butter, then cook at 250°F for 2 more hours.
Make the BBQ Sauce: Simmer ketchup, apple cider vinegar, brown sugar, molasses, Worcestershire sauce, Dijon mustard, smoked paprika, garlic powder, onion powder, black pepper, cayenne, and butter until thickened.
Finish the Ribs: Unwrap the ribs, apply BBQ sauce, and cook for 1 hour to caramelize. Rest for 10 minutes before serving.