1. Add plain flour, salt, baking powder, sugar and hand whisk together to combine in a medium bowl. Set aside.
2. In a heat-safe bowl, add water and milk. Microwave for 20 sec at 600W. Add dry yeast and a dash of sugar (extra), mix. Let it sit for a few minutes.
3. In a mixing bowl of your stand mixder, add the flour mixture (Step1) and the milk mixture (Step2) and lard. Mix at the lowest speed for 10 minutes, scraping down the sides of the bowl as needed.
4. Transfer the dough to a lightly floured bowl, wrap it in plastic wrap and let it rest for about 1hr or until the dough doubles in volume.
Prepare the filling :
5. Meanwhile, let's prepare the filling. In a small bowl, add all the seaoning ingredient (not corn starch yet) and mix. In a skillet, heat 1 tbsp oil and fry onion until wilted. Then add char siu and the seasoning sauce over the onion. Cook for a few minutes. Pour the dissolved corn starch with water and thicken the sauce. Let it cool and set aside.
Put it all together :
6. Transfer the dough onto a lightly floured surface. Using a rolling pin, roll the dough to a circle shape, divide the dough into 12 equal pieces. Shape each piece into a ball, cover with a damp cloth and let them sit for 15min (bench time).
7. Take one ball at a time (cover the remaining ones with a damp cloth to keep them from drying). Roll out the dough ball using a rolling pin until it becomes 10cm in diameter. The center of the dough should be thicker than the edge.
8. Scoop 1/12 of filling and place it in the middle of the dough. Wrap and close the dough up to seal from the top. Place the bun seam side down on an individually cut baking paper and cover with a damp towel. Repeat and finish all the buns in this way.
9. Place the buns in your steamer. Leave some space between the buns. Wrap the lid of the steamer with a cloth. Steam for about 15 min. Turn the heat off and leave the lid closed another 1 min before lifting it.