For the filling:
1 1/2 lbs pork shoulder, chopped into small pieces
1 small onion, finely minced
2 cloves of garlic, finely minced
4 tbsp sugar
4 tbsp soy sauce
3 tbsp oyster sauce
1 tbsp hoisin sauce
2 tbsp cornstarch mixed with 1/4 C cold water
1. Saute onions and garlic in some oil until soft and translucent. Add pork and continue to saute until pork is no longer pink. Add everything but the cornstarch mixture and let simmer on low for 30 minutes or until pork is tender. Add the cornstarch mixture and simmer for 2 more minutes. Let cool completely.
For the buns (adapted from David Chang's Momofuku steamed buns recipe)
1 tbsp + 1 tsp yeast
1 1/2 C water, room temperature
4 1/2 C bread flour
6 tbsp sugar
3 tbsp milk powder
1 tbsp kosher salt
1/2 tsp baking powder
1/2 tsp baking soda
1/3 C vegetable shortening
1. Combine the yeast and water in the bowl of a stand mixer fitted with the dough hook. Add the rest of the ingredients and mix on low for 8-10 minutes. The dough should be smooth and not too sticky. Place in a lightly oiled bowl, cover and let rise in a warm place for 1 1/2 hours.
2. Punch down the dough and turn onto a clean work surface. Divide into ping pong ball sized pieces. Place on a baking sheet, cover and let rest for 30 minutes. In the meantime, cut some 3-inch squares of parchment paper.
3. Flatten, then roll each ball into 4-inch wide circle. Fill with about 2 tbsp of filling. Pinch the dough to enclose the filling. Place a square of parchment on the bottom.
4. Place in a bamboo steamer and steam for 12 minutes. After steaming, you can let the siopao cool completely and freeze them. To re-heat, either microwave for about 2 minutes or re-steam.