Dough:
Pork Filling:
Mix cabbage and salt in a large bowl thoroughly and let stand for 10 minutes. Squeeze out water and return to bowl.
Place pork belly in food processor and process until finely ground; add cabbage, shiitake, scallions, and garlic and pulse 2-3 times until incorporated.
Add remaining ingredients and refrigerate up to 48 hours before using.
Mix flour, cornstarch, salt, sugar, baking powder, and yeast in a large bowl. Warm the milk to lukewarm in microwave or on stovetop. Mix half the milk into the flour, incorporate fully, and mix in the other half. Knead dough until smooth, oil the bowl, return the dough to the bowl and cover to rest for 30 minutes.
Cut the dough into quarters. Roll each quarter out into a 6-inch long snake and cut into 4 equal pieces.
Roll a piece out into a disc about 3½" in diameter. Place 1 Tbsp of filling at the center, fold the dough up and pleat/pinch around the perimeter until it's sealed: Christine Ho has an excellent YouTube video demonstrating the process. Do not worry if it looks like all the pleats are on one side of the bao: when you pinch off the top and fluff it up, the classic bao shape will form.
Repeat until all bao are assembled.
At this point, extra bao can be spread on parchment paper on a cookie sheet in the freezer and frozen, then put into ziplock bags for long-term storage.
Place 2 Tbsp oil in a frying pan on medium-high heat. Once it's heated, put fresh or frozen bao in for ~30 seconds (until bottom is seared), then add 1/4 c. water to pan and cover with a lid. Leave for 5 minutes (fresh bao) or 7 minutes (if the bao came from the freezer), then remove lid and use a spatula to transfer bao to serving plates.
Serve with a soy/vinegar/ginger dipping sauce or mix it up with a sweet chili, hot mustard, or other sauce of your choice.