For the dough:
1 tsp active dry yeast
3 1/2 cups cake flour
1 tbsp sugar
1 tsp baking powder
2 tbsp diced lard or vegetable shortening
Filing:
1 tbsp canola oil
3 scallions, white parts only, finely chopped
1 1/2 cups diced roast pork
3 tbsp soy sauce
3 tbsp oyster sauce
1 tbsp sugar
1 tsp cornstarch
For the dough: Combine yeast 1 1/4 cups water heated to 115 in a bowl; let sit until foamy, about 10 minutes. Combine flour, sugar, and baking powder in the bowl of a stand mixer fitted with a paddle. Add yeast mixture, and mix on low speed; add lard one piece at a time, increase speed to medium, and continue mixing until dough forms into a ball, about 5 minutes. Remove bowl from mixer, cover with plastic wrap, and let sit until doubled in size, about 2 hours.