Mix the flour, water, 1 egg, milk, sugar and yeast in a bowl. Add it into the bread machine to knead for 10 minutes until smooth.
Add the butter into the dough and mix well. And continue kneading it for another 10 minutes at medium speed.
Wrapped the dough in a fresh-keeping bag to seal the fermentation to twice the size. (The fermentation environment is controlled at a temperature of about 28°C and a humidity of 70 %.)
Add the pork, 1/2 teaspoon of salt, pepper, onion, vegetable oil in a bowl and mix well. Put it in the refrigerator for storage and set aside.
Transfer the dough on a workbench. And add some flour and knead it for 10 minutes until smooth. Divide the dough into 12-14 equal-sized pieces.
And press small doughs down to around wrapper. Scoop 1 tablespoon of pork filling on the dough wrapper center. Seal the buns.
Place the steamed buns on the baking tray for 40 minutes to 1 hour at room temperature until fermentation to twice the size. (The temperature of the fermentation environment is controlled at about 38°C and the humidity is 85%.)
Brush egg washes on the surface of buns and sprinkles some sesame seeds on it. (optional)
Preheat your oven at this time to 180°C /350°.Bake the buns for 20-25 minutes or until golden brown. Serve.