Preheat your smoker to 280°F and get a pellet tube smoldering inside the smoker. Fill the water pan with water.
Remove the silver skin from the back of each rack of ribs. Coat all sides with yellow mustard, then season generously with pork rub. Let the ribs rest for 10–15 minutes.
Place the ribs in the smoker and cook for 2.5–3 hours. While smoking, spritz the ribs 2–3 times with a mixture of equal parts apple cider vinegar and water, plus 1–2 teaspoons of liquid smoke.
Remove the ribs from the smoker. For each rack, lay out two layers of aluminum foil. Add brown sugar, a few chunks of butter, and a drizzle of BBQ sauce and liquid smoke. Place the ribs meat-side down onto the mixture, then wrap tightly in foil.
Return the wrapped ribs to the smoker. Insert a probe if available and cook until the internal temperature reaches about 205°F, about 45 minutes to 2 hours.
Carefully unwrap the ribs and brush all sides with BBQ sauce. Place them back in the smoker for 10–12 minutes to set the sauce.
