Soak the mushrooms in warm water for 15 minutes. Then, discard the water. Cut and discard the mushroom stems. Slice the mushrooms into thin strips. Drain the bamboo shoots and slice them into thin strips. Using a sharp knife, cut the pork into thin slices, then cut these slices into thin strips about 4 centimeters long. Dilute the cornstarch in 3 tablespoons of cold water. Beat the egg lightly with the sesame oil. Finely chop the chives. In a large pot, combine the broth, soy sauce, mushrooms, bamboo shoots, and pork. Bring to a boil over high heat, then immediately reduce the heat. Cover and simmer for 3 minutes. Add pepper and vinegar. Stir the cornstarch to dissolve it completely, then add it to the soup. Stir the soup for a few seconds, then incorporate the beaten egg. Remove the soup from the heat and pour it into a soup tureen or individual bowls. Sprinkle with chopped chives. Serve immediately.