Mix the lukewarm water with sugars and add yeast and wait until yeast activates (bubbles).
In a large bowl combine all the ingredients (wet into dry) and knead until soft for about 5 - 10 minutes.
Knead through shortening and continue to knead until soft, smooth and elastic.
Shape it into a ball and cover the dough with a damp cloth and let it rest until the ball doubles in size.
Knock down the dough and transfer to a flour'ed surface and divide to 10 or 12 pieces.
Shape in a ball, flatten it and wrap each piece with about 30 grams of char siew and seal the bun up the top with a twist or plait.
Add some baking paper underneath and let is proof for about 30 minutes before steaming - the bao should grow in size a little.
Steam on high for about 8 - 10 minutes and leave to cool down (in high humidity if possible) to prevent cracks.