Wash the prawns and pat dry with paper towel. In a large bowl sprinkle prawns with salt and set aside for 15 minutes.
Meanwhile mix together the cornflour slurry, soy sauce, sugar, rice wine, tomato sauce and dark soy sauce to make the sauce.
In a large wok deep fry prawns in hot oil until crispy, about 1 to 2 minutes. Remove them with a slotted spoon and drain on paper towels.
Drain the oil from the wok leaving about 1 to 2 tablespoons. Heat the oil over high and when the oil is smoking hot add the garlic and ginger then stir until fragrant.
Return the prawns to the wok and pour the sauce over. Toss until the sauce has been mostly absorbed and sprinkle with spring onions. Mix well.
Turn onto a serving platter and serve immediately with rice.