To make starter dough: Mix all the ingredients into a dough and leave at room temperature for 24 hours, covered with a piece of cloth. It should become foamy and smell tangy and yeasty. If the starter dough is not going to be used within the next 24 hours, it will keep in the refrigerator for up to 2 months. To use, remove from the refrigerator 5 hours ahead.
To make dough A: Make a well in the middle of the Hong Kong flour and add in the starter dough. Pour in some water and mix in a circular motion, adding water bit by bit. Knead until smooth. Place the dough in a well-oiled bowl; turn dough so that the oiled surface is facing upwards. Wrap bowl with cling film and keep in a cool place for 10 hours.
To make dough B: Place the dough and sugar in a mixing bowl. Using the dough hook, mix on low speed for 1 minute, or until sugar is dissolved and the dough is smooth. Tip in half the Hong Kong flour and half the baking powder. Mix well. Add shortening and ammonia. Mix for 10 seconds and add the remaining flour and baking powder. Add water and mix until smooth. Set aside, covered. Allow to rest for 1 hour before use.
To prepare sauce: Heat the oil in
a wok and add the shallots, onion, ginger and spring onions. Fry for about 3 minutes until fragrant and golden. Turn off the heat and strain the oil. Return the oil to the wok. Over medium heat, stir-fry with the peanut butter and fermented soy bean paste. Add the combined sugar, oyster sauce, dark soy sauce and water. Bring to a boil and simmer for 3 minutes.
Turn up the heat and pour in the tapioca, chestnut and corn flours followed by the water. Keep stirring continuously, as the mixture will thicken very fast. Stir for about 2 minutes until smooth. Transfer to a bowl and seal top with a layer of oil. Leave to cool overnight.
To make filling: Mix all ingredients together.
To assemble: Roll out the dough to resemble a sausage. Cut into nuggets of 20-30g each. Flatten lightly with your palm. Hold the flattened dough with your thumb and index finger and roll out the dough with a dim sum rolling pin until it forms a circle about 5cm in diameter.
Scoop 1 heaped tablespoon (about 30g) of the filling onto the middle of a dough circle and pinch the edges together. Place the pau on a small square piece of parch.