Steps

1. Make the filling: Put the oil and onions in a hot wok and stir-fry for about 1 minute. Add all the other filling ingredients except the cornstarch and the pork. Cook for 30 seconds, then add the cornstarch-water mixture to form a thick sauce. Mix in the pork and allow to cool.
2. Make the dough: In a large bowl, combine the flour, sugar, and baking powder. Make a well in the middle, add the milk, and knead until all the flour is moist. Continue to knead the dough for several minutes. Add the melted lard or shortening and knead it into the dough. Continue kneading for about 5 to 10 minutes, until the dough is smooth and elastic; when ready, it will bounce back when poked with a finger. Cover and let rest for 15 to 30 minutes.
3. Roll the dough into a long rope shape. Using a sharp knife, cut the dough into 20 pieces. Roll each piece into a ball and flatten each ball into a round disk. The middle of each disk should be thinner than the edges.
4. Make the buns: Place a tablespoon of filling in the middle of each disk. Pinch the dough together with your fingers and shape into a ball. Put the pinched side down on a square of parchment paper.
5. Place the buns, including their paper squares, into a bamboo steamer or double boiler, paper sides down. Steam over high heat for 15 minutes, until done. Serve warm.