Heat your pellet grill or smoker to 225℉ with your choice of wood or pellets.
Cut the pork butt into cubes about 1½" thick. *See notes for deboning and cutting instructions.
Toss the pork cubes in a large mixing bowl with the yellow mustard and Worcestershire sauce until evenly coated. Then arrange on a large cooling rack, slightly spread out so that smoke can get to all sides of the pork. *I placed my cooling rack on top of a sheet tray that I lined with peach butcher paper to reduce clean-up, but you can place the cooling rack directly on the grill grates if you prefer. Evenly season the cubes of pork on all sides with about ¼ cup barbecue seasoning.
Place the cooling rack of pork cubes on the grill, close the lid, and smoke at 225℉ for about 3 to 3½ hours, or until the internal temperature of the pork reaches about 160℉. *I also used super smoke mode on my Traeger Woodridge Elite grill, and I flipped the pork pieces halfway through so that they smoked evenly.
When the pork is almost to 160°F, make the barbecue sauce mixture in an aluminum foil pan. Add barbecue sauce, brown sugar, apple cider vinegar, hot sauce, and sriracha to the pan and mix together well. *Adjust sweetness or heat to taste, based on your chosen barbecue sauce. Toss the cubes of smoked pork in the pan with the sauce until they are well coated.
Raise the grill temperature to 275°F. Arrange the glazed pork pieces into an even layer in the foil pan, and add the ½ stick butter, cut into thinly sliced medallions, across the top.
Return the glazed pork to the smoker, and close the lid. Cook at 275℉ until the internal temperature of the pork reaches about 200-203℉, tossing the pork butt burnt ends every 30-40 minutes to evenly coat in the sauce. *For us, this took about an additional 2½ hours, but cooking times will vary. Always cook to the proper internal temperature, not to a specific time.
When the pork butt burnt ends are tender to your liking, about 200-203℉ internal, transfer to a serving tray and allow to cool about 10-15 minutes before serving.