Mark 18 x 5cm squares on baking paper, spray the paper with oil and cut out squares.
Combine yeast, 1 tablespoon of the sugar and 1/4 cup of the water in a small bowl. Cover, leave in a warm place, 15 minutes, or until frothy.
Sift the flour and baking powder into a large bowl, stir in remaining sugar. Stir in the yeast mixture and remaining water, mix to a soft dough. Knead the dough on a floured surface for about 5 minutes or until the dough is smooth and elastic. Place the dough in an oiled bowl, cover, stand in a warm place for about 1 1/2 hours or until the dough has doubled in size.
Turn the dough onto a floured surface and knead for 3 minutes. Divide the dough into 18 pieces.
Make the filling before proceeding to the next step.
Roll 1 piece of dough into a 10cm round on a floured surface. Ease the piece of dough into a 1/4-cup capacity measuring cup. Place a level tablespoon of filling into centre, press the filling down. Dip your fingers in water, gather the dough together with fingers, twist at the top. Place a piece of the oiled paper over the join and place on baking paper-lined trays, paper side down. Repeat with the remaining dough and filling.
Cover buns, let stand in a warm place, 20 minutes, or until the buns have increased in size.
Make a cross in the top of the buns with a small, sharp knife. Steam buns on paper squares in batches in a large, covered bamboo steamer over a pan of boiling water, 15 minutes, or until the buns are puffed and cooked through. Serve with red vinegar and ginger, if desired.
Steamed pork buns filling
For the filling, cover mushrooms with boiling water in a small bowl, stand for 20 minutes. Drain the mushrooms, discard stems, chop the caps finely.
Heat oil in a medium frying pan, add garlic and mushrooms, cook, stirring until fragrant. Add pork, sauces and blended cornflour and water. Cook, stirring until mixture boils and thickens. Cool.