Put the pork belly in a roasting pan that holds it snugly, fat side up. Combine 1/2 cup salt and 1/2 cup sugar in a small bowl and rub all over the pork. Cover with plastic wrap and refrigerate for at least 6 hours, and no longer than 24.
Heat the oven to 450ºF. Discard any liquid that has accumulated in the roasting pan and put the pork belly in the oven. Cook for 1 hour, basting it with the rendered fat halfway through, until it's golden brown.
Turn the oven temperature down to 250ºF and cook until the pork is tender, another 1 hour and 15 minutes or so. Transfer the pork to a plate, decant the fat and the meat juices from the pan and reserve it for the buns. Allow the pork to cool slightly.
When it’s cool enough to handle, wrap the pork in plastic wrap or aluminum foil and put it in the fridge until it’s thoroughly chilled and firm. (You can skip this step if you’re pressed for time, but the only way to get neat, nice-looking slices is to chill the belly thoroughly before slicing it.)
Combine the cucumbers with the remaining sugar and salt in a small mixing bowl and toss to coat. Let sit for 5 to 10 minutes. Use right away or refrigerate for up to 4 hours.
When you're ready to make the buns, cut the pork belly into 1/2-inch-slices about 2 inches long. Warm them in a pan over medium heat for a minute or two, until soft and heated through. Use the pork right away.
Stir together the yeast and 1 1/2 cups room temperature water in the bowl of a stand mixer fitted with a dough hook. Add the flour, sugar, milk powder, salt, baking powder, baking soda and fat and mix on the lowest speed setting for 8 to 10 minutes. The dough should gather together into a ball on the hook. Lightly oil a large bowl and put the dough in it, turning it over to coat it with the oil. Cover the bowl with a dry kitchen towel and put it in a warm place and let the dough rise until it doubles in size, about 1 hour 15 minutes.
Punch the dough down and turn it out onto a clean work surface. Using a sharp knife, divide the dough in half, then divide each half into 5 equal pieces. Gently roll the pieces into logs, then cut each log into 5 pieces, making 50 pieces total. They should be about the size of a Ping-Pong ball and weigh about 25 grams each. Roll each piece into a ball and set them on baking sheets. Cover them loosely with plastic wrap and let them rise for 30 minutes. While they're rising, cut out fifty 4-inch squares of parchment paper.
After 30 minutes, use a rolling pin to roll each ball into a 4-inch-long oval. Brush lightly with vegetable oil, lay a chopstick horizontally across the center of the oval and fold the oval over onto itself to form a bun. Gently pull out the chopstick, leaving the bun folded, and transfer it to a square of parchment paper. Put it back under the plastic wrap and form the rest of the buns. Let the buns rest for 30 to 45 minutes: they will rise a little.
Set up a steamer on top of the stove. Working in batches so you don’t crowd the steamer, steam the buns on the parchment squares for 10 minutes. Remove the parchment. You can use the buns immediately or allow them to cool completely, then put them in plastic freezer bags and freeze for up to 2 months. Reheat frozen buns in a stove top steamer for 2 to 3 minutes, until puffy, soft, and warmed all the way through. Freeze half the buns in airtight bags for another time.
Open a warm bun and spread about 2 teaspoons of hoisin sauce on the inside. Add 2 pieces of pork belly, then a couple slices of pickle. Add a scattering of scallion and a squirt of sriracha if you like. Repeat with the remaining buns, and eat!