Pork buns aren't hard to make. Simply truss the meat, season it, and let it roast for four hours until it's mouth-wateringly crispy. You can roast the pork on a rotisserie over a grill or in a roasting pan in the oven. The fat in the skin "confits" the pork into a succulent, delicious filling for these crowd-pleasing steamed buns.
Rub both sides of the pork belly with the vegetable oil. Season with salt, pepper and Chinese five-spice powder.
Roll the belly into a cylinder, skin side out, and truss tightly with butcher's twine. Place on a spit and roast until tender, with a drip pan underneath to catch the rendered fat, 3½–4 hours. To crisp the skin, stop the spit from rotating so that the skin gets broiled and puffy (watch carefully, as this will happen quickly); then turn to crisp all sides. (Note: If you don't have a spit, place the trussed pork belly on a rack set inside a roasting pan, with the seam side down, and add about a cup of water to the bottom of the roasting pan. Cover with aluminum foil and roast at 325 degrees F for 3½ hours. Turn the oven up to 500 F, remove the foil, and let the skin crisp up for the last 30 minutes.)
Remove the belly from the spit, and allow it to rest outside the oven while you prepare the buns. Prepare a steam basket set over a wok partially filled with boiling water, or use a pasta insert elevated over a pot of water. Steam the buns to reheat, 5–10 minutes.
To portion the pork belly, remove the butcher's twine and slice with a serrated knife. Assemble the buns by layering hoisin sauce, sliced cucumber, scallion whites and a piece of the pork belly. Serve immediately.