Preparing the custard (can be done the day before):
Whisk eggs, milk, sugar, and salt together in a saucepan. Sift in flour and milk powder. Stir to combine everything
Put on the stove over low heat. Add butter. Keep stirring until the mixture gradually get thicken to a custard consistency. Remove from the heat. Cover with a cling wrap touching the custard to prevent a layer of skin from forming. Chill in the fridge until it's firm, about 2 hours or longer
When ready to use, divide into 12 equal portions and roll into round balls. Keep them covered in the refrigerator while you prepare the dough
Make the dough with a stand mixer:
Place the flour, starch, instant yeast, sugar, and cooking oil in a mixing bowl. If you are using a stand mixer, use a dough hook attachment. Gradually add in liquid. If the dough still a bit dry, add a bit of water or milk. Add more teaspoon by teaspoon until the dough comes together and continue to knead for 10-12 minutes until the dough is very stretchy and smooth
Make the dough by hands:
If you are kneading by hands, mix everything until you get a rough dough (it's not going to be smooth yet). Cover and rest it for 15 minutes. Then go back and knead it. You'll be surprised by how easier it is to knead it now. Knead until the dough is smooth and pliable, about 15-20 minutes. Add liquid as needed. At any point during kneading, you can always stop and rest the dough if you find it hard to knead. This is to relax the gluten. Then go back to knead again and you'll be surprised by the difference it makes by just resting the dough before kneading again. It's easier for you too
Rest the dough:
Cover and rest the dough for 15 minutes. This is not to proof the dough. It shouldn't double in size. This is to relax the gluten for easier shaping and rolling the dough
Wrapping:
Place the dough on a lightly floured surface. Divide equally into 12 pieces. Work with one piece at a time and cover the rest to prevent drying. Flatten the dough with your palm and use a small rolling pin to roll into a circle about 5-6 inches in diameter and the middle is slightly thicker. Don't roll the dough out too thin too
Place the custard filling ball on the center of the wrapper and gather all the sides to the middle and pinch to seal. Place it on a parchment paper seam side down. Continue with the rest and place the bao on a large tray cover with a large clean towel
Proofing:
Let the dough rise at room temperature or at a warm place. If it's winter where you are, you can use your oven "bread proof" function to let them proof in there, or simply turn on your oven to the lowest temperature and then turn off and after 15 minutes, place the shaped buns in there to let them proof. They won't necessarily double in size, but at least puff up to about 50% of the original size. This may take about 30 mins to 1 hour at a warm temperature. Don't go by the time however. The buns will feel lighter and when you push it gently with your finger, it will spring back slowly. These buns are ready to be steamed
Steaming:
Bring the water in a steamer to a boil. Wrap the lid with a cloth. This will prevent water dripping from the lid and create burn spots
LOWER THE HEAT TO MEDIUM. Place the buns in there, leaving about 1-inch space in between. Cover with a lid but leaving it about 1/4-inch gap for some steam to escape. Steam on medium heat for 10 minutes
Once the buns are done steaming, turn off the heat and wait for 5 minutes before opening the lid so the buns will not collapse or wrinkle due to sudden change in temperature.
Cooling:
Remove from the steamer and put on a cooling rack for them to cool down. This will prevent the steamed buns from getting wet and soggy at the bottom
The buns are the softest when they are warm. They will turn slightly harder once they have cooled down completely and that's normal. We don't add any additives or preservatives to our dough. A simple brief reheating in a microwave or steamer will restore its softness
Storing:
Once the steamed buns have cooled down completely, you can cover them with a cling wrap and keep them at room temperature to serve on the same day. Leftover can be wrapped in cling wrap and keep in the fridge for a day or two. For longer storage, freeze them. Place them on a baking sheet and put them in a freezer for about 1 hour. They won't be frozen yet, but firm enough. Transfer to a freezer bag and they won't stick to each other anymore. They can be kept in the freezer for up to 1 month for the best result. You can thaw them overnight in the fridge and then steam them for 5 minutes to reheat or steam them from frozen for 10 minutes or until heated through