Place the ribs in a large bowl and cover them with cold water. Stir and agitate the ribs to rinse them well. Pour out the water, and rinse the ribs again under cold running water. Once the water is clear, leave enough to submerge the ribs, and stir in the baking soda (this will tenderize the ribs and give them a cleaner flavor). Soak for 30 minutes to 1 hour. After soaking, rinse and drain the ribs thoroughly.
Transfer the ribs to a mixing bowl, along with the garlic, ginger, sugar, salt, white pepper, five spice powder, red fermented bean curd, Shaoxing wine, sesame oil, oyster sauce, and dark soy sauce. Mix to coat the ribs evenly. Cover and marinate in the refrigerator overnight, or at least 30 minutes.
Just before steaming, toss the ribs with the cornstarch, incorporating it evenly. The marinade should be clinging to the ribs, with little to no standing liquid.
Spread the ribs evenly across a shallow heatproof dish that will fit into your steamer. Bring water to a simmer in a metal steamer or wok with a steaming rack set in the bottom. Place the dish of ribs onto the rack, cover, and steam over medium heat for 15-20 minutes, until the ribs are opaque and cooked through. Garnish with chopped scallion, if using.