Sift together the flour, sugar, salt, baking soda and baking powder into a large bowl. Set aside.
Warm the pork fat together with the milk until it reaches 37C or is lukewarm.
Place the dough back into the bowl, cover with a damp cloth and set aside to proof for 1hr.
Meanwhile prepare some 10cm squares of baking paper for the buns to be steamed on.
Divide the dough into 8 portions and pat down to about 1cm thick. Sprinkle a little green onion over the dough. Using your fingers, gently stretch out each portion and give it a twist before twisting it over itself and pinching the ends together.
Place each portion, seam side down, onto a square piece of baking paper. Cover the buns with a damp paper towel and allow to proof for 30mins.
Place buns into the steamer tray, leaving room for them to grow and steam for 6 mins.
Carefully remove the lid without dripping any condensation back onto the buns. Remove the steamer tray from the heat and wait 1-2 minutes before transferring the buns to a plate, this allows some moisture to evaporate and the buns to firm up.