In a saucepan, combine the chicken stock, chili flakes or chili powder, salt, all-purpose soy sauce, mirin, Shaoxing cooking wine, grated ginger, and sugar.
Put the saucepan on medium heat and bring to a boil then add the chicken drumsticks.
Reduce the heat to low, cover, and simmer for 30 minutes or until the chicken is cooked and easily falls off the bone.
When the chicken is cooked, remove it from the pan, put it on a plate, and shred it into bite-size.
Return the chicken to the pan then add sliced mushrooms, spring onions, carrot, soaking liquid from the dry shiitake mushroom, and vermicelli pasta.
Bring the saucepan to a boil and simmer gently for another 10 minutes.
Add the cornflour slurry to the saucepan and stir until the soup has thickened.
Slowly drizzle in the egg mixture in a thin stream while you stir the soup continuously to create egg ribbons. Remove the pan from the heat.
Add the sesame oil, vinegar, and freshly ground white pepper, check the seasoning and add more salt, vinegar, or chili if needed.