Hoisin sauce
A thick, dark, sweet-and-salty Chinese seasoning sauce made from fermented soybeans.
Hoisin sauce is stocked in the Asian aisle of many larger supermarkets, and reliably in Asian grocers or online. Once opened, it keeps for many months in the fridge.
Substitute:
There is no true substitute. A makeshift mix of dark soy sauce, a little honey, a touch of peanut butter or miso, garlic and a pinch of five-spice powder matches the sweetness and salt, but not the dense, fermented depth or the sticky texture.