Place the mushrooms in a small bowl and cover with hot water. Meanwhile, add the chicken stock, salt, and soy sauce to a soup pot and bring to a boil. While waiting for the liquid to heat up, slice the bamboo in thin matchsticks and trim the fat off the pork and cut it in strips 1/2 inch long and 1/4 inch thick. Remove the mushrooms from the hot water; put the mushroom water in the soup pot. Slice the mushrooms thinly. Add the bamboo, mushrooms, and pork to the now-boiling soup. Reduce heat, cover, and simmer for 3 minutes.
Use about half the block of tofu - you want about 3/4 cup of tofu sliced in matchsticks. Add the tofu, pepper, and vinegar to the pot. Bring to a boil again. Add the cornstarch mixed with water to the pot, and stir until the mixture is thickened, about 2 to 3 minutes. Turn off the heat and add the egg, stirring gently.
Ladle the hot soup in bowls and garnish with a teaspoon of sesame oil and some chopped scallions.
This could easily be made vegetarian or vegan by substituting dashi or vegetable broth for the chicken stock and omitting the pork and/or the egg.
If you want more sourness, try another tablespoon of vinegar.
If you want it even thicker, try another tablespoon of corn starch mixed with water.
(Image: Kathryn Hill)