Sift the all-purpose flour, wheat starch, and powdered sugar into a large bowl and make a well in the center.
Add the yeast to the well, pour in the 80 ml lukewarm water, and gently dissolve the yeast.
Slowly incorporate the flour mixture into the liquid, then add 1 tablespoon oil and mix to form a shaggy dough.
Knead the dough by hand for 10–15 minutes until soft, smooth, and slightly shiny.
Cover the dough with a damp cloth and let it rise in a warm place for 60 minutes, until roughly doubled in size.
Dissolve 1 teaspoon baking powder in 1 teaspoon cold water, sprinkle it over the risen dough, and knead briefly until combined.
Shape the dough into a cylinder and divide it into 8 equal portions; roll each portion into a ball.
Prepare the filling by combining the diced chicken, finely sliced napa cabbage, softened and diced shiitake mushrooms, 1/4 tsp salt, 1/4 tsp chicken bouillon powder, 1 tsp fish sauce, 1/2 tsp sugar, 3 dashes white pepper, 1 tsp cornstarch, and 1 tbsp chopped scallion; mix until evenly combined.
Flatten each dough ball with a rolling pin into a 3-inch (7 cm) circle and place a portion of the filling in the center.
Wrap the dough around the filling, pinch and twist at the top to seal tightly, and place each bun on a 2" × 3" piece of parchment paper.
Arrange the buns in the steamer with about 1 inch (2.5 cm) between them and steam over high heat for 10 minutes.
Remove the buns from the steamer and serve immediately.
