Fluffy steamed buns filled with sweet barbecue pork make an irresistible dim sum treat. A family favorite packed with protein and bold Cantonese flavor.
Combine all-purpose flour, instant yeast, granulated sugar, and warm water in a large bowl; knead for 8 minutes, then let rest covered until doubled in size, 60–90 minutes.
Punch down the dough, add baking powder and vegetable oil, knead briefly, then divide dough into 6 portions and shape into balls.
Dice char siu and green onion; mix with hoisin sauce, oyster sauce, soy sauce, and sesame oil in a skillet over medium heat, stir for 2 minutes until fragrant.
Mix cornstarch and water, add to skillet, stir continuously over medium heat until filling thickens, about 2 minutes; transfer to a plate and cool completely.
Flatten each dough ball into a disk, spoon cooled pork filling into center, gather edges, and pinch tightly to seal.
Arrange buns on parchment squares in a steamer, cover, and let proof 15 minutes; steam over high heat for 10–12 minutes with lid slightly ajar to prevent soggy tops.