Make the black bean sauce. In a small pan over low heat, warm the oil, then add the fermented black beans and minced garlic. Fry gently, until the garlic just turns light golden. Pour into a heatproof bowl and set aside — the garlic will keep cooking in the hot oil, so don't let it fully colour in the pan or it may turn bitter.
Prep the ribs. Cut the spare ribs between the bones into individual pieces. Wash thoroughly and pat completely dry.
Season and massage. Add the salt, sugar, chicken bouillon powder, and soy sauce directly to the ribs. Massage everything in firmly until the surface of the meat feels sticky. This step sets up the ribs to hold and absorb as much liquid as possible.
Add water gradually. Pour in the water 4–5 tablespoons at a time, massaging well between each addition. At the end, use a circular motion to help the ribs absorb it. The ribs should look noticeably plump. Use all the water, or stop once they can't absorb any more.
Coat with cornstarch, then sauce. Add the cornstarch and gently fold until all the pieces are coated. Pour over the black bean and garlic mixture and mix to combine.
Arrange for steaming. Lay your cubed pumpkin or taro in a heatproof dish or steamer tray. Spread the ribs evenly on top in a single layer.
Steam for 30 minutes. Bring a pot of water to a boil, place the dish in the steamer, and steam over high heat for 30 minutes. Carefully remove, garnish with sliced green onion and chilli if you like, and serve immediately.