In a stand mixer, add all the ingredients for the dough expect butter. Knead for 8-10 minutes at low speed. Then place softened butter in , use lowest speed to knead for 1-2 minutes until the butter is well combined with the dough. Speed up the stand mixer and knead another 10 minutes until the dough becomes quite elastic.
Place the dough in a warm place, covered and wait until at least 2 times in size.
Crust topping
In a large bowl, use an electric mixer to cream the softened butter until smooth. Add sugar and continue to beat until fluffy. Mix egg liquid and shift cake flour. Mix everything well. Place in refrigerator for around 30 minutes until slightly hardened. So you can handle the crust dough easily soon.
Assemble the bun
On a clean surface, knead the dough again for minutes until the air is completely pinched out. Divide the dough into 8 equal portions. Covered and rest for 30 minutes until 1.5 times bigger.
Divide the crust dough into 8 portions too. Roll each one to a round wrapper. The wrapper can be a slightly sticky and you can roll it with the help of plastic wrappers. Place one wrapper on the board, then flat the crust portion, cover with another plastic wrapper.
Cover the crust over the bun (from the top side).
Add some cuts for a lovely appearance.
baking
Pre-heat to 200 degree C (395 degree F) firstly. Brush egg wash on top.
Brushing Bake for 8 minutes firstly and then turn the temperature of oven to 180 degree C (355 degree F) and bake for another 10 minutes.