In a stand mixer, add all the ingredients of the dough expect butter, knead on low speed for 5-7 minutes.
Add softened butter in; continue kneading for another 7 to 10 minutes until you get a smooth, elastic and soft dough. transfer the dough out to a slightly floured board after 5 minutes and smash it against the operating board, grab it up and smash again. Repeat this process for 2-3 minutes and shape into a round ball. This can help to strong the gluten. Place in a large bowl and cover with a plastic wrapper and set aside until it is doubled in size. If you need help for this process, check milk bread.
Knead the dough again to remove the air inside and then further divide into 12 portions. Take one out, press down and roll out to a larger wrapper. Spread a thin layer of sweet Mayonnaise and then place 1 tablespoon of pork sung in. Wrapper well and then re-shape into a ball. Place it in a lined baking tray. Repeat to finish all the others.
Important tip for the second proof : place the baking tray in your oven with a large bowl of hot water inside. Close your oven door and wait until all the balls are at least doubled in size.
Pre-heat the oven to 170 degree C (350 degree F) . Mix green onion with 1/4 teaspoon of ground white pepper, 1/4 teaspoon of salt and olive oil. Brush a thin layer of egg wash on surface and then spread the green onion on top.
Bake for 20 minutes on middle rack.
Notes
The Nutrition Facts is based on each single bread.