Score the fat on the pork belly, and season with salt & pepper about two hours before the cook.
Keep in the refrigerator and place on a rack so air can circulate all around.
Thirty minutes prior to going on the smoker rub all over with yellow mustard. You want a thin coat, so you may have to wipe some off.
Set your smoker to cook using indirect heat and preheat to 250°F. (I used Kingsford Charcoal Briquettes for fuel and Cherry Wood for smoke flavor.)
Place pork belly directly on the grill grate, and spritz with apple juice every hour.
When the meat hits an internal temperature of 190°F, (this took three hours for me) wrap it in aluminum foil, and place back on the smoker.
When it reaches an internal temperature of 203°F, remove it from the smoker, unwrap it and let it rest on a rack for 20 minutes. (I had a total of 7 hours and 20 minutes on the smoker, but don't go by time. Go by internal temp. It could go faster or slower depending on your conditions.)
While the meat is resting mix together the gochujang, hoisin sauce, soy sauce, ketchup, honey, Korean rice wine, rice vinegar, ginger, garlic, white pepper.
After the meat rests for 20 minutes, cube it and mix with ONLY HALF the sauce in an oven pan. (Keep the rest of the sauce for adding extra at the table.)
At this point you can continue, or cover it and keep in the refrigerator until your ready to finish.
In the oven, set the top rack on the second slot and put the cubed, sauced meat under the high broiler for about 2-4 mins if you're continuing on. If coming out of the refrigerator, it may take 5-8 minutes. Keep an eye on it and remove it when it looks right.
Serve with your favorite sides.