Mix the yeast into two tablespoons of the lukewarm water. Place the flour, baking powder and yeast mix in a bowl and gradually add in the 300g water. Use your fingers to bring the mixture together and make a dough. Knead the dough for about 6 minutes until it is smooth. Cover the bowl with a plastic bag or kitchen towel. Rest for 20-30 minutes till the dough rises and doubles in size.
For the filling, chop the napa cabbage very finely and add a pinch of salt. After five minutes squeeze all the water out of this mix, then mix it with the remaining filling ingredients.
To make the wrappers, cut the dough into three equal pieces. Roll one piece into a long cylinder, then cut off 10 pieces of dough the size of walnuts, about 30g each. Dust them lightly with flour. Stand them cut-side up and press down them into discs with the palm of your hand. Roll each disc into a circle about 9cm in diameter. With the edges thinner than the center.
Place a wrapper in one hand, put a tablespoon of the filling (25-30 g) in the center of the wrapper. Use your other hand to pinch the edge of the dough to enclose the filling. Fold the edges to the center around the filling while twisting so the bun is completed sealed.
Place all the buns in a steamer basket – make sure there is space between them, as they will expand.
Using a bamboo steamer, cook on a high head for 30 minutes. Turn off the heat without opening the lid and let the buns rest for five minutes before serving.