Preparation Steps:

• In a mixing bowl, combine the all-purpose flour, wheat starch, powdered sugar, and active dry yeast. Mix gently. Gradually add the lukewarm water and oil, mixing until a shaggy dough forms. Add baking powder and knead the dough on a lightly floured surface for about 5-7 minutes until it's smooth and elastic.
• Place the kneaded dough in a lightly oiled bowl, cover it with a damp cloth, and let it rise in a warm place for about 30-45 minutes, or until it has doubled in size.
• While the dough is rising, prepare the filling. In a large bowl, combine the diced chicken breast, finely sliced napa cabbage, diced shiitake mushrooms, salt, chicken bouillon powder, fish sauce, sugar, white pepper, cornstarch, and chopped scallion. Mix until all ingredients are well incorporated.
• Once the dough has risen, punch it down to release the air. Divide the dough into equal pieces, about 30g each. Roll each piece into a small circle, about 4 inches in diameter. Place a generous tablespoon of filling in the center of each circle.
• Gather the edges of the dough over the filling and pinch to seal tightly. Ensure there are no gaps for the filling to escape during steaming. Place each bun seam side down on a piece of parchment paper.
• Prepare your steamer by bringing water to a boil. Arrange the buns in the steamer basket, making sure to leave space between each bun. Cover and steam for about 15-20 minutes, or until the buns are puffed and cooked through.
• Carefully remove the buns from the steamer and let them cool for a few minutes. Serve warm, and enjoy the delightful combination of fluffy dough and savory filling!