Separate egg whites and yolks. Mix the egg yolks with the sugar.
Sift cornstarch and cake flour.
Mix well, no flour particles are left.
Add milk and mix well.
Heat on medium-low heat, and add unsalted butter.
When the butter is melted, pour in the egg yolk mixture.
Heat on medium-low heat, stirring with a spatula.
It will begin to set in a few minutes, stirring constantly with a spatula.
Stir until it forms a dough and becomes very fine, then it's done.
Place in a food container and cover with plastic wrap.
Cover the surface with plastic wrap, let cool, and store in the refrigerator.
Now start making the dough, add sugar to lukewarm milk. Add instant yeast.
Prepare all-purpose flour. Add the mixed yeast mixture to it.
Stir while pouring.
Add cooking oil, so the buns will look brighter with the oil added.
Knead into the dough by hand.
This is a medium-soft dough.
Knead the dough and transfer it to the mat to knead.
Knead to a strip, then fold, and repeat this until the dough is smooth.
Then roll it into a long strip.
Divide the dough into 14 parts, about 41 grams each.
Cover with plastic wrap to prevent air-drying
Take out the dough and roll it around.
Cover with plastic wrap when all done.
Take out the custard filling.
Roll into long strips.
Divide into 14 parts, about 25 grams each.
Roll out each part into a round.
Take out a small dough, flatten it, and remember to sprinkle more flour on the mat to prevent sticking.
Turn it over, smooth side down.
Roll into a round sheet with a thick center and thin sides.
Put a custard filling in the middle.
Wrap it up like a bun.
Put it on the mat and roll it up a bit so that it won't get misshaped when steamed.
Put it on baking paper and put it inside the steamer.
Remember to leave some space between the buns.
Cover and let it ferment, it will take about 1 hour.
Check the state of the dough after fermentation, it is light and flimsy when picked up, it will slowly spring back when pinched.
Steam for 10 minutes after the water boils.
Turn off the heat and simmer for 5 minutes before removing the lid.