Make the Sauce: Mix all ingredients for the sauce (except the pork shoulder) together in a small microwave-safe bowl.
Place the pork slices in a ziplock bag or bowl, and pour in half the sauce. Mix well and let the pork marinate for at least 30 minutes, and up to 24 hours in the refrigerator.
Remove pork from the marinade and place in a single layer in your air fryer basket.
Set the air fryer to 390F for 15 minutes, flipping and basting the pork half way through.
While the Char siu is cooking, place the reserved half of the sauce in the microwave and cook on high for 45-60 seconds stirring every 15 seconds. You are doing this to thicken the sauce a little. Since it has honey in it, be careful to not overcook otherwise you will have char siu toffee (ask me how I know).
Using a meat thermometer, ensure your pork has reached an internal temperature of 145F.
Remove the pork to a cutting board and let it rest for 10 minutes.
Brush with the thickened, cooked sauce and serve.
Oven Instructions
Cut the pork tenderloin into 4-5 evenly sized pieces and place in gallon ziplock bag
Pour in half of the char siu sauce and zip up the back securely
Knead the sauce into the tenderloin and put in the fridge for at least 4 hours and up to 24 hours
Heat oven to 375 F
Line a baking sheet with foil and place a rack on top of it
Place the tenderloin pieces on top of the foil and dab on the remaining sauce on each side of the meat
Bake for 25 minutes on one side
Cook for another 15-20 minutes, or until a thermometer reads 145 degrees
Dab on to the meat and flip it over, dab on this side as well. Don't try to spread it, just dab it so it goes on thicker
Put the meat back in and cook for another 20-25 minutes, or until a thermometer reads 165 degrees