Make the dough: In a large bowl, whisk together the Bao Flour, White Sugar, Instant Yeast and Baking Powder.
Add warm (not hot) Water and Neutral Oil, plus a pinch of Salt; mix until a shaggy dough forms, then knead on a lightly floured surface until smooth and elastic (about 8–10 minutes).
Place the dough in a lightly oiled bowl, cover, and let rise in a warm spot until doubled in size, about 45–60 minutes.
Prepare the filling: Dice the Cantonese Char Siu into small cubes. In a skillet or saucepan, combine the pork with Oyster Sauce, Hoisin Sauce, Light Soy Sauce, Dark Soy Sauce and a little extra White Sugar to taste.
Add a splash of Shaoxing Wine, then season with Chinese Five-Spice Powder and Ground White Pepper. Gently simmer until the flavors meld and the sauce is slightly reduced, 3–5 minutes.
Thicken the sauce by stirring a slurry of Cornstarch with a little cold water into the pan until glossy; remove from heat and finish with a few drops of Toasted Sesame Oil. Let the filling cool to near room temperature.
Shape the buns: Punch down the risen dough and divide into even pieces (about 12–16, depending on desired size). Flatten each piece into a 3–4 inch circle, place a spoonful of the cooled char siu filling in the center, then gather and pleat the edges to seal the bun.
Place each sealed bun seam-side down on a small square of parchment. Arrange buns in a steamer basket, leaving space for them to expand.
Proof the buns for a short second rise, about 15–20 minutes, until puffy. Meanwhile bring water in your steamer pot to a rolling boil.
Steam the buns over high heat for 12–15 minutes (adjust time for size). After steaming, turn off the heat and let the buns sit in the closed steamer for 2 minutes before opening to prevent collapse.
Serve warm from the steamer. Char Siu Bao are best eaten the day they’re made, but you can freeze cooled buns and re-steam from frozen when needed.
