Set a rack into a wok or deep pan. Fill with 2 1/2 inches (6cm) water and bring it to a simmer. Rub the salmon fillets with the salt and sesame oil and place on a heatproof plate. Place the plate with the salmon on top of the rack, cover tightly and steam for 6 minutes.
While the salmon is steaming, heat a wok or large frying pan until it is hot, then add the oil, black beans, garlic and ginger. Stir-fry the mixture for 1 minute, then add the spring onions, soy sauces, water and simmer for 1 minutes. Add the cornflour mixture and stir the sauce until it thickens. When the salmon is cooked, pour the hot sauce over the salmon and serve at once.