General tips
Marinate in maltose, hoisin, soy, and Chinese five-spice for at least 12 h. Maltose is the secret - it's a malt sugar that gives char siu its distinctive lacquered, sticky-sweet glaze. Honey is a passable substitute but maltose is the real thing. Find it at any Asian market.
Cook at 145°F for 6 h for the firm, sliceable texture that defines char siu. Lower temps make the pork too soft to crisp under high heat; higher temps cook out the marinade flavor. The 145°F mark is the modern restaurant standard.
Glaze and torch in three coats. After the bath, brush with reserved marinade, torch, brush again, torch again, brush a third time and torch. Each layer caramelizes onto the previous, building the deep mahogany color and the sticky-sweet exterior.
Use pork shoulder, not loin. Char siu needs fat to render under the high-heat finish, and pork shoulder has the right marbling. Loin is too lean and dries out fast. Shoulder also stands up to the strong marinade flavors better.
Anti-tip: don't slice the meat hot. Char siu should rest at room temperature for 15 min before slicing - the meat firms up enough to give you clean slices that show the pink interior and the lacquered edge. Hot slicing tears the meat.
Tender tips
Marinate overnight for maximum flavor - the long marination is traditional and important
Don't add marinade to sous vide bags - it makes pork too salty and prevents proper texture
Pat pork very dry before finishing - moisture prevents caramelization and char
Caramelized tips
This firmer texture is perfect for dicing into small cubes for fried rice or ramen
Still needs careful basting during finishing - the glaze is what makes char siu special
Holds together better for chopping and meal prep than tender 140°F version