1Heat your smoker to 250°F with hickory, oak or apple wood chunks. Let the smoke settle to a thin, clean blue before the meat goes in.
2Pat the pork cubes dry with paper towels. Toss them with the mustard in a large bowl until every surface is lightly coated.
3Stir together the rub: 3 tbsp brown sugar, smoked paprika, kosher salt, black pepper, garlic powder, onion powder, chili powder and cayenne. Sprinkle it over the cubes and toss well to coat on all sides.
4Arrange the cubes directly on the smoker grate with a little space between them. Smoke at 250°F for about 3 hours, until the bark is dark and set and the internal temperature reaches about 175°F.
5While the pork smokes, whisk the ketchup, cider vinegar, 1/4 cup brown sugar, honey and Worcestershire in a bowl until smooth.
6Move the cubes into the foil pan. Pour the sauce over them, scatter the butter pieces on top, and stir gently so every piece is coated.
7Cover the pan tightly with foil and return it to the smoker. Cook for about 90 minutes, until a probe slides into a cube with almost no resistance (around 200 to 205°F).
8Remove the foil, stir the cubes, and cook uncovered for another 30 to 45 minutes, stirring once or twice, until the sauce is thick and tacky and clings to the pork in a glaze.
9Rest the pan for 10 minutes so the glaze sets up, then serve the burnt ends hot, with pickles and white bread if you like.