Mix soy sauce, sugar, Chinese cooking wine, black peppercorns, star anise, and cinnamon sticks in a bowl.

Marinate the pork belly in the soy mixture overnight.

Put the pork and marinade in a sous vide bag and cook for 8 hours at 170F.

Thicken the cooking juices in the bag with a cornstarch slurry.

Slice the pork into thin pieces.

Mix all the dry ingredients in a bowl.

Mix in water, milk, and oil. Stir until a ball of dough forms.

Knead the dough on a lightly-floured surface for about 10 minutes.

Leave the dough to rest for an hour in a lightly oiled bowl.

Roll the dough into a log. Cut into 5 equal pieces.

Flatten each portion of dough to a round shape with a rolling pin.

Fold each dough in half over a lightly oiled chopstick. Carefully pull chopstick out.

Leave the shaped dough to rest for another 20 minutes in an oiled baking pan.

Steam the buns for 10 minutes.

Stuff the bao buns with pork slices, thickened sauce, and cucumber slices.