Mix the yeast and sugar with the lukewarm water and let it sit until bubbles appear on the surface. Into a separate bowl add the flour and form a well. Add the salt and baking soda to the flour along the rim of the bowl and not in the well. Pour the yeast liquid into the well and form a dough.
Knead the dough for about 10 minutes until smooth. Then let it rest for two hours in a bowl covered with a damp tea towel.
Divide the dough into 12 equal portions. Form balls and then flatten out with your hands to a thickness of about 1 centimeter.
Line your steaming baskets with baking paper. Also put small pieces of baking paper on the bao rounds so that they cover half of the circle. Then fold the other half of the round over the baking paper. Place them in the steaming baskets above boiling water and steam for about 10 minutes.