Combine the all purpose flour, wheat starch, baking powder, yeast, and sugar into the mixing bowl with the dough hook attachment. Set to the lowest / stirring speed until all the mixture is evenly distributed.
Turn the mixer speed up slightly to level 2.
Combine the water and oil. Slowly add the liquid into the mixer.
Turn up the speed to level 5. When the dough turns into a ball (and releases from the sides), continue to knead on medium (about level 5) for 5 more minutes. The ball of dough should look smooth with no lumps and completely detach from the sides of the bowl.
Remove the dough from the mixer, and use your hands to form it into a ball. On a clean surface, hover your hands over the ball while making a heart with your thumbs and fingers, and make circular movements with the ball to get a tight, smooth surface.
Place the ball into a bowl, cover it with a lid, and let it rest for at least 5 minutes.
Poke the ball of dough, if it leaves a mark from your finger, you can begin to roll it into balls.
Transfer the ball onto a clean surface, remove the excess gas from the dough by kneading it a few times with the heel of your hand.
Dough kneading
Use a rolling pin to make the dough into a rectangular shape, about 14x9 inches with ¼ inch thickness (if you scaled the recipe, you will need to adjust this part). Sprinkle a little flour if it’s sticking too much to the surface.
Make a trifold by folding the top half towards the middle, then folding the bottom half over the middle fold. Turn the rectangle 90 degrees and roll out again into a 14x9 rectangle. Repeat once more. The dough should have a very smooth texture with no air bubbles.
Rolling
Take your pointer finger and press down on the top edge of the dough in an upward motion. You should get a very thin edge. This will help with sealing the roll.
Use the brush and dip it into the filtered water. Brush on the top of the dough.
Take the bottom edge of the dough and fold it upwards to begin the roll.
Make the roll as tight as possible without air bubbles and continue until you reach the top.
You should end up with a log. Seal the edges by lightly pinching the edge to the log. Use the palm of your hands to roll out the log slightly so the layers will stick together. The log should be about 16 inches long.
Use a sharp knife to cut it into 2 inch thick pieces.
Cut parchment paper in about 3x3 squares. Put each piece on a parchment square.
Add warm water to a saucepot and place the steamer on top. Add the mantou pieces to the steamer, making sure each one is separate by at least 1 ½ inches--these will expand. Cover with a lid and let rest for 20 minutes.
After proofing, uncover and check the dough, they should be at least 1 ½ times bigger. Cover again with the lid and turn on the stove to high heat. When the water begins to boil, lower to medium-low and continue to steam for 20 minutes. Add additional water if it runs too low. Note: If you cannot fit all of the proofed bao into your steamer at once, place the rest in the fridge to prevent them from proofing more. Chilling them should slow down the proofing process. Remove them from the fridge about 5 minutes before steaming them to get them to room temperature.
After 20 minutes, remove the pot from the stove and let the steamer rest for 4 minutes before opening the lid. This will prevent the mantou from deflating due to temperature shock.
Uncover the lid and serve the mantou immediately.
Notes
IMPORTANT: I included ingredients by weight because this is more accurate for baking. If you don't have a scale and must use volumetric measurements, I've included those in the recipe above too, but please note the volumetric measurement numbers will not change if you scale up or down the servings made in this recipe and you will need to do your own math to figure it out.