Let's start with the water roux/tong zhong. Mix the water and flour in a small pot, stir it until there are no lumps. Put it on small heat, keep stirring until it becomes a paste. Set aside.
Warm the milk up to around body temperature, leave the instant yeast in it to dissolve for 5 mins.
In a separate bowl, put all the dry ingredients (bread flour, salt, and sugar) together and mix it well.
In a big mixing bowl, put the milk mixture, dry ingredients, water roux/tong zhong and egg in it. Mix it up with a spoon until it forms a dough. Either use a mixer or knead it until it becomes smooth. (Take around 10 mins when I use a hand mixer)
Add in the butter and knead it until it mixed in and the dough becomes smooth again.
Sprinkle the mixing bowl with some flour, and leave the dough in it covered with cling film to rise for 45 mins or until it doubles the size.
Take the dough out on a surface sprinkled with flour. Press out all the air, knead it back to a ball and leave it on the table to rest for 15 mins.
After that, weigh the dough and equally divide it into 6 portions. Roll them one by one like a ball, flatten it to a round shape that is around 0.3cm thick using a rolling pin. Add the Char Siu Fillings in the middle, use a small spoon to press it in gently, and gradually close the opening. Make sure to use the fingertip to press it tightly and it's close completely. With the opening facing down, put your hand gently on top of it (with the fingers around it) and rotate it on a floured surface for a few rounds. Then put it on a baking tray with a baking sheet. Repeat it for the rest.
Leave the dough in a warm place to rise for 1 hour. I like to leave it in a closed oven with some warm water (not hot - will be too much steam!)
While waiting for the rise, prepare the crispy surface. (See below)
When the 1 hour is up, preheat the oven to 200 degrees Celcius. Pipe the crispy surface butter paste in a swirl motion on top of the bread dough, tightly and evenly without gaps. Make sure they are around evenly, if it's too heavy on one side, it will drop when it's in the oven.
Put it in the oven and bake for 12-15 mins, or when the surface starts turning golden brown. Then it is ready!