Add the dough ingredients except for butter into a mixing bowl, and mix with a dough hook slowly for 1 minute. Then change to medium speed, and mix for 3 minutes until a dough is formed.
Add the butter gradually and mix for 5 minutes on medium until fully developed. It should be elastic, smooth, not sticky, and slightly not touch the sides of the mixing bowl.
Put a cloth over the mixing bowl with the dough and leave it under the sun (or a warm place) for about an hour until the dough has doubled in volume. If you live in an area with a cold climate, I suggest warming up your oven as a substitute for the sun’s warmth.
Punch and knead the dough. Divide into portions of about 60 grams. Let the portions rest for 15 minutes at room temperature.
Flatten the bread into a round shape and place a tablespoon of pork floss in the middle of the dough. Seal the edges of the round dough to make a bun shape.
Either grease a tray or line it with baking/parchment/greaseproof paper and put the buns here. Put a cloth over the tray and buns and leave the dough for 45 minutes.
Brush the beaten egg over each piece of bread so it turns a light golden brown, and sprinkle sesame seeds on top of the buns.
Bake in an oven preheated at 170 degrees Celsius for about 15 to 18 minutes until it has turned golden.
Place the buns on a wire rack and let the buns cool.