In a large mixing bowl, combine plain flour, cake flour, wheat starch, icing sugar, baking powder, instant yeast, and water. Use a chopstick or ladle to mix and gather the ingredients into a dough. Transfer the dough to a flat surface and add cooking oil (in 3 batches). Knead the dough gently until it feels smooth, soft, and supple, about 10 minutes. Avoid overkneading.
Form the dough into a ball and gently press it. It should slowly bounce back. Next, roll it into a log shape and divide it into 10 equal portions. Each portion weighs about 43-44 grams. Cover the portions with a cloth to prevent them from drying out. Rest for 5 minutes.
Take one portion and shape it into a ball by pulling the edges towards the center. Ensure the surface is smooth. Lightly dust your working surface with flour and place the dough on it, smooth side down. Then flatten the dough using the palm of your hand. Then use a rolling pin to flatten it further to about 10 cm in diameter.
Place 2 heaps tablespoons of pork filling in the center of the flattened dough. Pleat the edges of the dough to seal the filling inside completely. Place the bun on a piece of baking paper. Gently press the sides to shape it into a round bun. Transfer the bun to a steaming basket and cover the basket with a lid. Repeat the process with the remaining dough portions.
Then let it proof until the dough look plump and smooth. The time depends on the type of flour used and your room temperature.