1.
To make the filling – in a large frying pan, heat the oil until hot and sauté the spring onions until fragrant, about 2-3 minutes. Add the remaining ingredients and stir until thick and bubbly. When cool, cover and allow to chill in the refrigerator overnight.
2.
To make the dough – in the bowl of a stand mixer, place the oil, sugar, yeast, egg and water. With the dough hook attachment, on medium speed, gradually add the flour until a soft and elastic dough is formed. Continue to knead on a medium speed for an additional 10 minutes. Cover the bowl with cling film, place into the refrigerator and allow for an overnight cold prove.
3.
Take out the dough and sit at room temperature for about 1 hour. Punch the dough down and divide into 16 equal sized pieces, covering them with a damp cloth.
4.
Flatten a dough ball slightly, making the edges thinner than the middle and fill it with approximately 1 tablespoon of filling, seal and place onto a piece of greaseproof paper seal down. Repeat with the rest of the dough and filling.
5.
Cover the filled bao with a cloth and allow to prove for 1-1 ½ hours or until puffy and well risen.
6.
Preheat the oven to 180 C / Gas 4 and bake the buns in the middle of the oven for 15-20 minutes (turning half way to ensure even browning).
7.
As soon as the baos come out of the oven, brush with melted butter and leave to cool slightly before devouring. Enjoy!