Char Siu doesn't refer to a sauce particularly, but the final product after this "Chinese barbecue sauce" is applied to pork that is hung onto fork skewers and roasted. Still, there's a fairly common base set of ingredients including hoisin, honey, soy sauce, sherry, Chinese five spice powder that imparts the ubiquitous flavor and glossy sheen to Char Siu.
The resulting sauce is sweet and salty with a distinct Asian flavor from the fermented soy in the hoisin, and a spiced kick from the five spice powder—a mixture of anise, fennel, cinnamon, cloves, and Sichuan peppercorns.
You can add food coloring to give that standard red hue, but last time I did that, my pork looked like some mutant, unnaturally bright red form of meat. Since then I've been sticking to the sauce sans food coloring, which results in a more earthy red color that I actually find more visually appealing.
Use this sauce as a marinade on pork belly, ribs, loin—any piece of pork you want to give an Asian barbecue flavor to, but it's also great as a dip or spread as well.
1/3 cup hoisin sauce
1/4 cup honey
1/4 cup soy sauce
3 tablespoons dry sherry
1 teaspoon Chinese five spice powder
In a small bowl, mix together Hoisin sauce, honey, soy sauce, sherry, and five spice powder.