put the flour, sugar, instant yeast, baking powder, and salt, together in the bowl of a stand mixer
mix the water and oil together
pour over the dry ingredients and mix on low with the bread hook
let the mixer knead until the dough comes together, it will look dry at first but don't worry it will work out
once the dough comes together let it knead for 2-3 minutes until the dough is soft and silky
put the dough in an oiled bowl, cover, and let rise in a warm place until doubled in size
while the dough rises start the filling
heat a heavy pot over medium heat
chop the pork belly into bite-sized pieces
brown it in the hot pan
add the onions, and mushrooms and let the thing cook over medium heat stirring to keep it from burning
when the pork is browned add the water and scrape up any crispy bits
mix the ketchup, hoisin, vinegar, and smoke flavor together and pour into the pan
cook and stir over low heat until the meat is glazed and the sauce thick, set aside until ready to use
cut 10 4 inch squares out of parchment paper or foil
once the dough has double gently punch push the air out of it and divide it into 10 equal pieces
roll each piece into a ball and place on it's own square of parchment
brush with a little milk and cover with plastic wrap
let rise 15 minutes until puffy
when puffy gently flatten one ball by hand leaving a bump in the middle
place a teaspoon or so of crispy onions on the dome
put one TBSP of filling over the onions
bring the sides up and over the filling, pinching it shut, and sealing
place it back on the square and cover with plastic wrap
repeat with remaining balls of dough until all the buns are filled
leave covered while you start the steamer
get the steamer set up and steaming over high heat
once it's going steam as many as you can without crowding them in, they need several inches between them for expansion
cover and leave covered for 15 minutes
don't open the lid to see what's happening
once the timer goes off remove the steamer from the heat and carefully remove the buns to a baking sheet
serve while still hot