Peel the cucumbers and slice very thinly. Place in a dish and add the soy sauce, vinegar, green onions, sugar, chili powder and sesame oil. Set aside for 1 hour, then add the chicken stock. Toast the sesame seeds in a small pan over medium heat until they turn golden and begin to pop, then grind finely. Transfer the soup to a tureen and sprinkle on the sesame seeds. Serve at room temperature or slightly chilled. Typed by Syd Bigger.