Peel The Prawns And Discard the shells. Using a small sharp knife split the prawns partially and remove the fine digestive cord. Pat the prawns dry with paper towels and combine with the salt, cornstarch and sesame oil and mix well. Heat a wok or large frying pan until it is hot. Add the oil and prawns. Stir-fry for 1 minute. Remove the prawns with a slotted spoon. In the remaining oil, add the chiles, black beans, garlic and green onions. Stir-fry for 20 seconds and add the vinegar, soy sauce and sugar. Stir in the cornstarch mixture and return the prawns to the wok. Cook for another 2 minutes and serve at once.