Whisk or stir together broth (fine to use bouillon or soup base), cornstarch, soy sauce, vinegar (white or seasoned vinegar can also be used) and cayenne in pot. Bring to boil over high heat, stirring constantly, until slightly thickened. Egg is optional and can be reconstituted from freeze-dried: Beat egg and gradually pour egg into boiling soup, stirring constantly. Serve immediately, garnish with chopped green onion.