Pork Belly Buns (Gua Bao)
For the Pork:
1/2 cup kosher salt
1/2 cup sugar
4 1/2 cups water, divided
2 1/2 lbs boneless skinless pork belly, cut into four pieces lengthwise
1/2 cup chicken stock
For the Buns:
1 teaspoon active dry yeast
1 teaspoon granulated sugar
1/2 cup water
1/2 cup all purpose flour
1 1/2 cups cake flour
1 1/8 teaspoon baking powder
1/4 cup sugar
2 1/2 teaspoons melted unsalted butter
1/4 teaspoon salt
Chili sauce or sriracha, optional
Brine the pork: Combine the salt, sugar, and 4 cups water until dissolved. Place the pork belly in a resealable plastic bag, pour in the brine, and seal. Place the bag in a bowl and place in the refrigerator to brine for at least 12 hours, or overnight.
Make the dough: Dissolve the yeast and sugar in warm water (105 degrees), and let it stand for 10 minutes. Sift together the all purpose and cake flour, as well as the baking powder. Set aside. To the yeast mixture, add the melted butter and salt. Stir in the flour to form a soft dough. Knead until smooth (5-10 minutes). Place in an oiled bowl, and cover lightly with plastic wrap, and let rise in a warm place until doubled in size. This should take about 1-2 hours.
Roast the pork: Preheat the oven to 300 degrees F, with a rack placed in the center of the oven. Remove the pork from the brine, and place it in a baking pan, fat side up. Pour the chicken broth and 1/2 cup water into the pan. Cover tightly with foil and roast the pork belly for about 2 1/2 hours, or until tender. Remove the foil and increase the oven temperature to 450 degrees. Roast for another 20 minutes or so, or until the fat on the pork belly is golden. Remove from the oven and let it cool for 30 minutes, then chill in the refrigerator for 1 hour. Slice the chilled pork belly crosswise in 1/4 inch pieces. Return to the pan juices and place it in the refrigerator to chill again. This can be done the day before serving, if you wish.
Steam the buns: When you are ready to make the buns, roll the dough into a log measuring about 16 inches long. Slice into 16 1 inch pieces, then set aside, covering with plastic wrap to keep the dough from drying out. Take each piece of dough and make a round ball, then roll out to about 3×6 inch ovals. Lightly brush the dough with canola oil, fold lightly then set aside to rise, covered, for another 20 minutes.
Prepare your steam bath in a wok or deep sided skillet with a steamer. The water should be about 1/2 inch below the steaming rack. Line with waxed or parchment paper and steam the buns, making sure they have room to expand between (do not crowd the steamer!). Work in batches as necessary. The buns can also be prepared in advance, just keep covered in the refrigerator and re-steam when ready to serve.
To serve: Preheat the oven to 350 degrees. Heat the pork belly in the baking pan with the pan juices until hot. Coat the inside of the steamed buns with hoisin sauce, and layer with cucumbers, pork belly, chile sauce if using, scallions and cilantro. Serve immediately.