Activate yeast: In a bowl, whisk warm milk and sugar. Sprinkle in instant yeast and let it sit for 3–5 minutes until foamy.
Mix dry ingredients: In a large bowl, combine 2 1/2 cups flour, baking powder, and salt.
Bring it together: Add the yeast mixture and oil to the dry ingredients. Stir until a shaggy dough forms.
Knead: Turn onto a lightly floured surface and knead 6–8 minutes until smooth and elastic. Add small dustings of flour only if sticky. The dough should feel soft, not stiff.
First rise: Place in a lightly oiled bowl, cover, and let it rise in a warm spot until doubled, about 1 hour.
Preheat a skillet over medium heat. Add oil, then garlic and ginger. Cook 30 seconds until fragrant.
Add ground beef. Break it up and cook until mostly browned, 4–5 minutes. Drain excess fat if needed.
Stir in soy sauce, oyster sauce, hoisin, sesame oil, white pepper, and scallions. Add cabbage if using.
Sprinkle in cornstarch and cook 1–2 minutes until the mixture looks glossy and lightly thickened. The filling should be moist but not wet.
Cool completely before filling the buns. Warm filling can tear dough.
Deflate the risen dough gently. Divide into 12 equal pieces. Keep them covered with a damp towel.
Working one at a time, roll each piece into a 3 1/2 to 4-inch round. Make the edges slightly thinner than the center.
Add about 1 1/2 tablespoons filling to the center.
Pleat and seal: Hold the round in your palm, pinch small pleats around the edge, and twist to seal at the top. If pleats feel tricky, simply fold and pinch the edges together and roll the seam under.
Place each bun on a parchment square, seam-side up for a classic bao look.
Arrange buns in the steamer baskets, leaving space to expand.
Cover with the steamer lid and let them proof 20–30 minutes until slightly puffy. Do not overproof.
Bring water in your steamer base to a steady simmer. Do not let it boil aggressively.
Steam buns over medium heat for 10–12 minutes. Keep the lid on the entire time.
Turn off the heat and wait 2 minutes before lifting the lid to prevent sudden temperature shock and collapse.
Serve hot, with chili crisp, soy, or black vinegar on the side.
