Char siu so (叉烧酥, BBQ pork flaky pastry) is a dim sum preparation of a laminated, flaky Chinese pastry shell filled with diced char siu (Cantonese barbecue pork). The pastry is made using a Chinese laminated pastry technique (水油皮, shui you pi, water-oil pastry) — a water-based outer dough laminated with a shortening-based inner dough — that produces a distinctive honeycomb-like flaky structure when baked. The exterior of a well-made char siu so is snow-white, delicate, and shatteringly thin; each layer peeling away from the others as the pastry is eaten.
Two-dough lamination: Water dough (水皮, shui pi): 150g plain flour + 30g icing sugar + 60g lard + 60ml water. Mix to a smooth, pliable dough. Oil dough (油皮, you pi): 100g plain flour + 60g lard. Mix to a crumbly, cohesive dough. The lamination: Wrap the oil dough inside the water dough (the oil dough forms the inner layer). Roll into a rectangle. Fold in thirds (letter fold). Rotate 90 degrees. Roll and fold again — repeat 3-4 times. The result is a dough with dozens of alternating water and oil dough layers. Divide into 16 portions. The char siu filling: Finely dice the char siu. Combine with oyster sauce, honey, hoisin sauce, a small amount of cornstarch. The filling should be sticky and well-glazed. Shaping: Roll each portion into a circle. Place a spoonful of filling in the center. Fold the pastry around the filling, pinching and crimping the seam to seal. Place seam-side down. Baking: 200C for 15-18 minutes until the pastry is golden. Do not overbake — the pastry should be pale-gold.
Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009)