For the filling:
Put the rack of spareribs into a large resealable plastic bag. Pour in the hoisin sauce, close the bag and toss gently to coat. Put the bag into a bowl and marinate the ribs in the refrigerator for 3 hours or up to overnight.
Preheat the oven to 300 degrees F.
Put the ribs into a sheet pan along with all the marinade. Cover the pan with aluminum foil, slide it into the oven and bake until the meat easily separates from the bone, about 2 hours. Baste them every 30 minutes during the cooking process. Remove the ribs from the oven and let them rest.
When cool enough to handle, pull the rib meat from the bones and coarsely chop. Then, in a large bowl, toss the pork together with the cabbage and scallions. If the mixture seems too dry, add in a bit more hoisin sauce.
For the dough:
Sprinkle the yeast over the warm water in a small bowl. After a few minutes, the yeast will begin to foam and bubble.
In a large bowl, whisk together the flour, sugar, and baking powder. Make a well in the center of the dry ingredients and add the warm milk, canola oil, and the activated yeast. Work the dough to produce a shaggy mass. Turn it out onto a floured board or countertop and knead until the dough becomes smooth, soft, and pliable, about 5 to 10 minutes. Put it into a lightly oiled bowl and turn it over once to coat. Cover the bowl in plastic wrap and allow the dough to rise undisturbed for 40 minutes to 1 hour. The mass should double in volume.
For the assembly:
To form the buns, remove the dough from the bowl and roll it into 1 long rope. Cut the rope into 12 equal portions. Roll each piece and set them aside in a bowl. Using a rolling pin, flatten each ball into 4-inch circles, leaving the center slightly thicker then its edges. Put a heaping tablespoon of the filling into the center of each disk and pinch the tops closed to seal.
Arrange the buns on a parchment lined sheet tray and cover them with a clean kitchen towel. Alternatively, place the buns into a large 3-level bamboo steamer lined with parchment paper, 4 buns per level. Allow the buns to rest for 10 minutes.
In the meantime, fill a bamboo or stainless steel steamer with water and bring it to a boil. Cut 12 (4-inch) rounds from some parchment paper. When ready to cook, place the buns on the parchment rounds and into the steamer.
Do not overcrowd your steamer. Steam the buns in batches if necessary. Steam for 12 to 15 minutes, then remove the buns from the heat and serve them warm.