Combine Hongkong/bao flour, powdered/icing sugar, and salt in a large bowl. Then sift into a separate large bowl. Make a well in the center.
Pour lukewarm water into the well. Add dry yeast granules and let it dissolve, about 2 minutes. Stir to mix. Add vegetable oil and continue mixing to form a soft dough.
Remove dough from the bowl and continue to knead on a lightly floured surface for 5 to 6 minutes until a soft, smooth, non-sticky dough forms.
Put dough back into the bowl and cover with a damp towel. Allow dough to rise in a warm place for about 1 hour or until double in size.
Heat vegetable oil in a medium size pan. Saute onion and garlic for 1 to 2 minutes. Add char siu and continue to cook for 2 minutes.
Add char siu glaze. When liquid comes to a boil, reduce heat and simmer for 2 to 3 minutes.
Pour in corn starch mixture and allow sauce to thicken. Turn off stove. Stir in green onions. Set aside and allow filling to cool completely.
Preparing Dough Wrappers
When dough has more or less doubled in size, gently punch down and remove dough from the bowl. Sprinkle baking powder over the dough and continue to knead until all baking powder is incorporated. Moisten your hands with water and continue to knead if the dough appears to be a little dry.
Return dough to the bowl. Cover with damp towel and let it rest for 10 minutes.
When dough is ready, remove from bowl. Cut dough into two. Roll a portion of the dough into a cylinder. Cut into 8 equal pieces. Do the same for the other portion of dough.
Flatten a piece of the dough into a disk. Using a small rolling pin, roll out the edges to form a wrapper of about 3 inches in diameter with a slightly thicker center and tapered edges.
Prepare a few dough wrappers at a time. Wrap baos with filling before preparing more dough wrappers. Keep dough in the bowl covered with damp towel to prevent it from drying out.
Prepare 16 pieces of 2-in x 2-in parchment paper.
Place a piece of dough wrapper on your palm and a rounded tablespoon of filling in the middle of wrapper. Gather and pleat the edges. Then pinch and twist to seal.
Place seam side up onto parchment paper in a steaming rack. Repeat with remaining dough.
Prepare a steamer and allow water to come to a boil. Place buns in the steaming rack. If you are using a metal rack and lid, place a towel over the rack. Omit towel if you are using a bamboo rack. Cover with lid and steam over rapidly boiling water for 12 minutes. Steam in batches if necessary.
Remove and serve immediately.
Leftover steamed buns may be placed in zip top plastic bag and kept in the freezer. Steam frozen buns for 8 to 10 minutes before serving.