Rub all the surfaces with mayonnaise. This will help ensure a tacky surface for the rub to stick and it also helps to tenderize the meat.
Apply enough rub on all exposed surfaces until you cannot see the meat underneath (a medium-heavy coat).
Let sit uncovered in the refrigerator for about 30 minutes.
Heat a smoker to 275 degrees F (135 degrees C). Once it gets to temperature, add three tennis-sized chunks of hickory wood and three tennis-sized chunks of apple wood.
Smoke the short ribs until the crust or bark is formed, 2 to 3 hours. Once the crust starts to form (about 90 minutes), spray the ribs with plain tap water from a spray bottle. Repeat the spraying every 15 minutes.
Once the crust has set, remove the ribs from the smoker and place them in the middle of a sheet of aluminum foil.
Sprinkle the dark brown sugar evenly on the ribs. Drizzle about 2 tablespoons of mango nectar over the ribs. Turn the ribs over and repeat the brown sugar and mango nectar on the other side.
Wrap the ribs tightly in the foil and return to the pit.
Check for tenderness using a bamboo skewer after 45 minutes by opening the foil. The skewer will go through like a muffin when it is done. If not yet tender, close the foil and check in 15 minutes. Keep checking until the meat is probe tender.
Remove when done and open the foil to vent (this step is very important else the ribs will overcook).
Once the ribs have cooled a bit, remove from the foil pouch. Mix some molasses with the au jus in the foil and brush the molasses/au jus on the ribs. Then apply a thin coat of your favorite barbecue sauce. Give it a dust of cayenne pepper before you slice and serve.