Make your parchment pieces of paper. Divide parchment paper in 16 pieces of 5x5 inches. Set aside.
Create the dough. Add baking powder and flour into a bowl, and mix. Set aside
Warm the milk to (100-110 degree F). Then add the yeast. Mix. Allow the mixture to sit for 10 minutes or so until it begins to foam. Add sugar and vegetable oil and mix until well-combined.
Put the wet ingredients in an enormous mixing bowl. Then add the dry items from Step 1. Mix with your hands. Once all the ingredients have come together, transfer it onto a lightly floured surface and then knead it until it is smooth for about 10 minutes. You can add more flour if the dough appears too moist. The dough is well-kneaded enough that you can poke it, and it then slowly bounces back. After that you can transfer it to an oil-coated bowl and put it over an absorbent towel. Allow it to rest in a warm location for at least an hour.
As the dough starts in the process of rising you can prepare the rest of your ingredients. Boil the eggs and cut them into four pieces. Slice your Chinese sausages. Set aside.
Create this pork filling. To make the pork filling, mix ground pork with wood ear mushrooms, chopped yellow onions, fish sauce oyster sauce, salt sugar, and pepper. Mix until well together. Divide the filling of pork into 16 equal round pieces and put them aside.
After the dough has finished proofing, cut in 16 equally sized pieces. Then, roll each piece into a small circle which is 4-4.5 inches across. The edges on the outside should be less than the middle. Be sure that the center isn't too thin.
Add 1 pork filling, one slice of Chinese sausage and 1 egg in the middle of filling. Wrap the filling around the edge of the dough. Crimp and fold the dough until it overflows with filling. Examples are shown in the images above. Put each bao onto a sheet parchment paper.
Set up your steamer pan. Bring the water to a simmer and add a pinch in rice vinegar. Be sure to maintain an even temperature throughout this process.
Banh bao are cooked for 15 to 17 minutes. You should ensure that there is at minimum 1 inch room between every bao because they expand.