Place the flour and starch, instant yeast, sugar, salt, and oil in a mixing bowl. If you are using a stand mixer, use a dough hook attachment. Add cold milk or room temperature water. Start with 180 ml and add more until you can form a dough. You will most likely need to add a bit more milk/water. I would rather you start with the lowest amount of liquid and add more teaspoon by teaspoon until the dough comes together. Continue to knead at low speed (speed 2 on KA) for 12-15 minutes or until the dough is smooth, silky and elastic. There is no need to reach a window-pane stage
If you knead by hand, mix 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. The kneading motion is like washing cloth with hands. At any point during kneading, you can always stop and rest the dough if you find it hard to knead and so much resistant. 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. You want the dough that is smooth, silky, and elastic. Just to warn you that it's going to take some time to knead by hands
SHAPING AND FILLING:
Divide the red bean paste into 12 equal portions. Roughly shape them into ball as that makes it easier to wrap later
Lightly dust your work surface with some flour. Flatten the dough with your palm and then use a rolling pin to roll the sides thinner to form a circle with a thicker middle part. This is to support the weight of the filling
FOR ROUND SHAPE: Place the dou sha filling ball in the middle and gather the edge to enclose into a round ball and place the seam side down. Cup the dough with both palms and move the dough in a circular motion to shape it taller. This part is important so your steamed buns come out tall instead of spreading to the side after steaming
Place on a piece of parchment paper. Lightly dab the surface of the bun with some milk using your finger to smooth the surface. Loosely cover with a saran wrapper to prevent drying. Work with another dough and do the same
IF YOU WANT TO PLEAT SOME PATTERNS: Place the dou sha filling ball in the middle. Create a fold around the edge and then pinch to seal. Cup the dough with both palms and move the dough in a circular motion to shape it taller. This part is important so your steamed buns come out tall instead of spreading to the side after steaming
Place on a piece of parchment paper. You don't need to dab the surface with milk if you have patterns. Loosely cover with a saran wrapper to prevent drying. Work with another dough and do the same
PROOFING:
Let the shaped and filled buns proof until about 50% double its original size at a warm place. They don't have to double in size. This may take about 30 minutes to one hour, but please don't go by the time, just observe to make sure the buns have puffed up to half its original size before steaming and when you gently push on the dough, it slowly bounces back. If it bounces back immediately, the dough needs longer proofing time
STEAMING:
Most likely your steamer won't be able to accommodate steaming all buns at one go. You may need to steam in 2-3 batches, which means, the rest of the batches will sit longer and continue to proof while waiting for the steamer. Not good for the buns! Here's what you can do: Make sure the cover them with plastic wrap and then place them in the refrigerator to slow down the yeast activity or halt it all together until they are ready to be steamed
Prepare your steamer. Bring the water in the steamer to a boil. Wrap the lid of your steamer (like wrapping a gift basically) with a clean kitchen towel and secure with rubber band/ string or whatever you have on the top. This is to prevent the condensation from the steam drops back to your bao and this will create "burn spots" later. When I steam with a bamboo basket, the condensation is not a problem
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 15 minutes. This is especially important if you see that you have overproof the steamed buns (the steamed buns have doubled in size and they have widened to the side). As long as you steam them like this, the buns will not wrinkle and collapse later when you take them out from the steamer
Turn off the heat. Do not open the lid of the steamer. Let the buns sit there for 5 minutes like this. The buns will not sink or wrinkle due to the sudden change in temperature
STORING:
If you make extra and plan to store them, place them on a baking sheet, not touching each other, and then put the entire tray inside the freezer for about 1 hour. They will harden, but not completely frozen yet. Transfer to a freezer bag and they will not stick to each other anymore. Try not to keep for more than 1 month
REHEATING:
They can go straight from freezer to steamer when you ready to eat them. Steam on high heat for 5 minutes and they are as good as new