Black bean sauce (豆豉酱, dou chi jiang) is one of the four or five most fundamental sauce preparations of Cantonese cooking — built on fermented black beans (douchi) fried with garlic, ginger, and sometimes fresh chilli, then combined with oyster sauce, soy sauce, and a small amount of Shaoxing wine. It appears as the sauce base for steamed pork ribs, steamed fish, stir-fried bitter melon, and numerous other Cantonese preparations. The commercial Lee Kum Kee black bean sauce (豆豉醬) is a reasonable starting point, but freshly made black bean sauce from quality douchi is incomparably better.
Homemade black bean sauce: 50g douchi (Yang Jiang variety preferred) — rinse briefly, roughly mash or chop. 4 garlic cloves — finely minced. 1 tbsp fresh ginger — finely minced. 2 fresh red chillis (optional) — finely sliced. Fry garlic and ginger in 3 tbsp neutral oil until fragrant (30 seconds). Add the douchi — fry 2-3 minutes over medium heat until the douchi begins to release its aroma and the oil turns slightly darker. Add 1 tbsp Shaoxing wine, 1 tbsp oyster sauce, 1 tbsp light soy, 1 tsp sugar. Stir to combine. Remove from heat. Store refrigerated in a sealed jar. Use as a sauce base: When using for steamed dishes (pork ribs, fish), place 1-2 tbsp of black bean sauce on top of the protein before steaming. The sauce flavours the protein as it steams. For stir-fries, use 1-2 tbsp as the sauce component added in the middle of the stir-fry.
Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009); Fuchsia Dunlop, Land of Fish and Rice (2016)