In a large mixing bowl, combine plain flour, cornflour, caster sugar, instant dried yeast, and baking powder. Gradually add warm water and vegetable oil, stirring until a sticky dough forms.

Place the kneaded dough into a lightly greased bowl and cover with a damp cloth. Allow to rise in a warm location for 60-90 minutes until doubled in size.

Once risen, punch down the dough to release air. Divide into portions, roll into balls, and flatten into round disks about 4-5 inches in diameter. Allow to rise again for 30 minutes.

Boil water in a steamer. Place buns in the steamer, cover, and steam for 10-12 minutes until puffy and cooked through.

Prepare the chicken by cutting into bite-sized pieces. In a bowl, mix chicken with soy sauce, Shaoxing cooking wine, sea salt, white pepper, and egg white. Marinate for at least 30 minutes.

Coat marinated chicken with additional flour. Heat vegetable oil in a pan and fry chicken pieces for 5-7 minutes until golden brown and crispy.

In a saucepan, combine soy sauce, gochujang, muscovado sugar, honey, and minced garlic. Simmer for about 5 minutes until sauce thickens.

To assemble, open a steamed bao, stuff with crispy chicken, drizzle with spicy sauce, and top with cucumber and spring onions.