Flip your pork butt so the fat cap is facing up and use a knife to score the fat in a criss-cross pattern.
Slather the entire pork butt in a thin layer of yellow mustard to act as a binder, then season it with the rub.
Preheat your pellet grill to 225°F.
Once your grill is preheated, place the pork directly on the grates with the fat cap facing down (or towards the heat source).
Let it smoke for 8-10 hours until it reaches an internal temperature of 165°F and the bark on the outside is firm to the touch.
Transfer the pork butt to an aluminum pan and add the apple juice to the bottom of the pan.
Place the pats of butter all over the top of the pork butt, then re-season the whole thing with the BBQ rub.
Cover the pan with aluminum foil and place it back on the smoker for another 6-8 hours, or until the internal temperature reaches between 195°F and 200°F. The temperature probe should slide in and out with little resistance.
Let the pork butt rest for at least 30 minutes, then shred it up and serve immediately.