Soak the noodles in hot water for 15 Minutes. When soft, drain well and discard the water. Cut into 7.5cm (3in) lengths, using scissors or a knife. Set aside.
Pour the stock into a wok and bring to the boil. Add the beef, beanthread noodles, soy sauces, rice wine or sherry, sugar, salt and pepper and turn the heat down to moderate. Gently simmer for about 4 Minutes.
Add the cornflour mixture and continue to cook, stirring constantly, for another 3 Minutes or until the soup has slightly thickened.
Finally, add the sesame oil and give the soup several good stirs.
Turn into a soup tureen or individual bowls and serve at once.