Remove the skin from the pork belly if present. Cut the pork belly into 1.5-inch cubes as uniformly as possible. Chill in the freezer for 20 minutes if the fat makes it slippery to cut.
Coat each cube lightly with yellow mustard on all sides. Combine all dry rub ingredients and apply generously to all six sides of each cube, pressing the rub in. Let rest at room temperature 15–30 minutes.
Preheat your smoker to 250°F using cherry, apple, or pecan wood.
Place the seasoned pork belly cubes on a wire cooling rack set over a sheet pan, leaving space between each cube. Place the rack on the smoker grates.
Smoke uncovered at 250°F for 2.5 to 3 hours until a firm, dark bark forms and internal temperature reaches 165–175°F. At the 90-minute mark, lightly spritz with apple cider vinegar if the surface looks dry.
Transfer smoked cubes to a disposable aluminum half-pan. Scatter cubed butter over the top, sprinkle with brown sugar, and drizzle with honey. Toss gently to coat, then spread in a single layer.
Cover the pan tightly with aluminum foil and return to the smoker at 250°F for 60 to 90 minutes, until internal temperature reaches 200–205°F and a thermometer probe slides in with no resistance (probe tender).
Remove the foil. Carefully drain most of the braising liquid from the pan. Toss the cubes with BBQ sauce and apple cider vinegar, coating each piece evenly.
Return the pan to the smoker uncovered for 15–20 minutes until the glaze tightens and caramelizes into a glossy, sticky coating. Watch carefully — the glaze can burn quickly. Pull when the exterior of each cube looks tacky and glistening.
Remove from smoker and let rest 10–15 minutes before serving. Serve directly from the pan with toothpicks, or use as a topping for buns, tacos, mac and cheese, or nachos.