In a large bowl, combine 1/2 cup warm water, 2 teaspoons sugar, 2 teaspoons yeast and a teaspoon of flour. Let stand 20 minutes until the yeast is foamy. Add the rest of the 270g flour, stir with chopsticks and then knead to a smooth dough. Cover with cling wrap and let stand for 4-6 hours.
Meanwhile, make the filling: Mix the 150ml water, 1 tablespoon sugar, 1 tablespoon oyster sauce, 1 tablespoon sesame oil, white pepper to taste and 1 tablespoon flour in a small saucepan. Heat, stirring constantly, until thickened. Remove and chill. When cool, stir in the barbecue pork.
To finish the dough, sieve together the baking powder, 60g cake flour and 70g cornflour. In a jug combine the 80g sugar, 40ml oil and 30ml water and stir well.
When the dough has risen for 4-6 hours, stir in the sugar, oil and water mix and knead well. Add the sieved-together baking powder, cake flour and cornflour. Knead into a smooth and non-sticky ball. Cover with an upside-down bowl and let stand for 10 minutes.
Divide the dough into 12 to 15 pieces and roll into balls. Flatten the balls into rounds. Spoon a large tablespoon of pork filling into the centre of each and seal by pinching the edges up like a sack. It's fine if the top is a bit thick, that will help the tops look like opening flowers when they are steamed.
Place each bun on a small piece of baking paper and place in bamboo steamers. Let rise for 5 minutes before steaming, covered over boiling water for 12 to 15 minutes until cooked.
You can add many different flavourings to the filling - try green onion, minced ginger, five spice, garlic or soy sauce.