Sift the all-purpose flour, wheat starch, and powdered sugar into a large mixing bowl.
Make a well in the center and add the yeast and 80 ml lukewarm water; gently dissolve the yeast, then mix into the flour mixture and add 1 tablespoon oil.
Knead the dough by hand for 10–15 minutes until soft, smooth, and slightly shiny.
Cover with a damp cloth and let rise for 60 minutes or until noticeably expanded.
Dissolve 1 teaspoon baking powder in 1 teaspoon cold water and sprinkle over the dough, then knead briefly to incorporate.
Roll the dough into a cylinder and divide into 8 equal portions; shape each portion into a ball.
Prepare the filling: combine the chopped chicken, sliced napa cabbage, rehydrated diced shiitake mushrooms, 1/4 teaspoon salt, 1/4 teaspoon chicken bouillon powder, 1 teaspoon fish sauce, 1/2 teaspoon sugar, 3 dashes white pepper, 1 teaspoon cornstarch, and 1 tablespoon 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 filling in the center.
Wrap the dough around the filling, pinch and twist to seal the top tightly, and place each bun on a 2" x 3" (5 x 7 cm) piece of parchment paper.
Arrange buns in the steamer with about 1 inch (2.5 cm) gap between them and steam over high heat for 10 minutes; optionally add 1 teaspoon Chinese white vinegar to the steaming water to help keep buns white.
Remove buns from the steamer and serve immediately.