To Make the Bun Dough
Sift the two types of flour together along with the icing sugar. Place the sifted flours and icing sugar together in a mixing bowl and make a well in the middle.
Combine the vinegar, lukewarm water and yeast together before pouring them slowly in to the middle of the flour well and mixing.
Add in the vegetable oil and knead the dough for about 10 minutes or until the dough is soft and smooth.
Cover the dough with a damp towel and let it rise for about 30 minutes and has doubled in size.
Dissolve baking powder in cold water and pour over the dough while kneading it until they are both well combined.
Divide the dough in to roughly 16 equally sized portions and roll out in to 3 inch circles.
To Make the Pork Filling
While the dough is rising, heat oil in a pan and saute the onions for about 2 minutes to soften them before adding in the char siu, sugar, oyster and soy sauce and sesame oil.
Stir the ingredients well until they are evenly mixed. If the dish isn’t red enough, you can choose to add 1 to 2 drops of red food colouring in to the filling.
Mix together the water and cornflour before adding them in to the pork filling and continuing to cook until the sauce has thickened to a gravy like consistency.
Remove the pan from the heat and let cool until the assembly of the buns.
To Make the Pork Buns
Place a heaped tablespoon of the pork filling in to the centre of one of the bun circles and wrap it in a circular pleat like manner. Continue until the bun has been sealed.
Place a small circle or square of baking paper on to the bottom of the bun to stop it from sticking to the steamer before placing them in to it.
Continue to fold the rest of the buns remembering to leave a gap in between each bun in the steamer.
Bring a saucepan or wok of water to a roiling bowl before placing the steamer on top. Steam the buns for about 12 minutes ensuring that the top of the steamer remains closed to trap all of the moisture.
After the buns have cooked, allow them to cool slightly before serving!