Trim & cube: Pat pork belly dry. Trim any excessively thick fat. Cut into roughly 1.5–2” cubes (uniform size for even cooking).
Make the rub: In a bowl combine 3 tbsp brown sugar, 2 tbsp kosher salt, 1 tbsp smoked paprika, 1 tbsp black pepper, 1 tbsp garlic powder, 1 tbsp onion powder, 1 tsp ground mustard, 1 tsp chili powder. Mix well.
Season: Lightly brush cubes with a little neutral oil so the rub sticks. Generously coat each cube in the rub. Let sit 15–30 minutes (or refrigerate covered up to overnight).
Preheat smoker/grill: Bring smoker or grill (indirect) to 225–250°F (107–121°C). Use apple or cherry wood for a sweet smoke; add pecan or hickory if you like deeper smoke. Place a pan of liquid (water or apple juice) in the smoker to help humidity.
Smoke, stage 1: Place cubes on a rack with space between pieces. Smoke until deep mahogany bark forms and internal temp ~165°F (about 1.5–2.5 hours). Optional: spritz with apple juice + apple cider vinegar every 45–60 minutes.
Prepare glaze: In a saucepan over low heat combine 1 cup barbecue sauce, ½ cup apple cider vinegar, ½ cup brown sugar, 4 tbsp butter, 2 tbsp honey/maple, 2 tbsp Worcestershire, and 1 tsp hot sauce (if using). Warm to dissolve sugar and marry flavors; do not aggressively boil.
Braise for tenderness: Transfer smoked cubes to a heavy foil pan. Pour enough glaze to coat and partially submerge the cubes (about 1–1½ cups). Cover tightly and return to smoker at 275°F (135°C). Braise 45–90 minutes, until internal temp reaches 195–205°F and cubes are fork-tender.
Caramelize finish: Remove foil, stir to coat. Return pan uncovered to smoker or oven at 300°F (149°C) for 20–40 minutes, until glaze reduces and becomes sticky and caramelized. For extra crust, increase heat to 350°F for the last 8–12 minutes while watching closely.
Rest & serve: Let rest 5–10 minutes. Serve warm on a platter with extra glaze on the side, or build sliders, tacos, or serve over mashed potatoes.