Sift flour into a mixing bowl. Add double-action baking powder and yeast. Stir well to combine. Make a well in the centre and add in vinegar and egg white. Pour in sugared water gradually and add in shortening. Mix and knead into a smooth and soft dough. Cover with a piece of cling film and leave aside to proof for 1-2 hours.
Roll out dough on a lightly floured surface into a long cylinder. Divide into10-12 equal portions. Roll out each portion into a thin round skin.
Add a tablespoon of filling onto the dough skin, then bring the edge of the skin up and pleat to seal the bun. Give it a twist and pinch off any excess dough at the last pleat. Place the bun on a small round of greaseproof paper with the pleated side up. Repeat to make more buns. Arrange the buns well apart in a bamboo basket and steam over rapidly boiling water for 10-12 minutes.
For the filling: Season chicken with ingredients (A) for 30 minutes. Heat oil and sesame oil in a wok and fry garlic until lightly golden. Sprinkle in cooking wine (optional). Add in marinated chicken and stir-fry well. Add mushrooms and water chestnuts. Mix in seasoning (B). Dish out and leave to cool completely.