Whisk together the low-gluten flour, baking soda, sugar and yeast for the dough. Add the lukewarm water and combine well. Like all baozi dough, the dough will be pretty soft at this point. Knead into a soft and smooth dough—but do not over-knead, as that may activate the gluten and result in less-fluffy baozi. Cover with a damp towel and let the dough proof to twice its size.If you are making this in the winter, or keep the house below 68°F (20°C), the dough will take longer to proof. To mitigate this problem, you can use your oven as a proofing box or even a microwave.
While the posubao dough is proofing, combine the lard and AP flour into a lard dough (see note). Set aside.
Dust flour on a flat, clean surface. Press on the posubao dough to let the air out. Then roll the dough out into a thin, rectangle shape. Using a scraper, apply a thin layer of the lard dough across the top and dust with a thin layer of AP flour. Then, starting from one edge, roll the dough into a long, thin roll. Divide into 2 long sections. Gently roll one of the logs to adjust the shape. Then, divide the log into 4-5 equal sections by hand. Repeat with the second log. Let the pieces rest for 10 minutes or so before starting to wrap, giving the wrapper time to relax so it will be easier to wrap the baozi.
Take one piece of the divided dough and fold the corners towards the middle to form a round ball shape. Then, press it with your palm to flatten the dough into an approximately 4-inch (10-cm) wide wrapper. Repeat with the rest.
Place 1/8th of the filling in the middle of a wrapper. Fold into a simple bun shape, with as few folds as possible. Be sure to make a tight closure on the top.
Transfer the buns to a steamer with a small piece of baking paper or cheesecloth on the bottom. Let the buns proof in a warm place for 15 minutes.
In a wok or other steamer-compatible wide pot, bring a couple inches of water to a boil on full heat. Once boiling, place the steaming racks on top. Set a timer for 10 minutes before turning off the heat.