To prepare the ingredients: Thinly slice the scallions. Crack the eggs into a bowl and whisk well. In another bowl, mix the cornstarch with 3 tablespoons water to make a slurry. Place the dried seaweed in a separate bowl, pour in the 2 cups warm water, and soak for about 1 minute. Rinse the seaweed under running water and drain well.
To start the soup: Pour the chicken stock into a pot over high heat. Add the salt, stir, and bring to a simmer. Stir the cornstarch slurry again, then slowly pour it into the stock while stirring. Cook until slightly thickened, then bring to a boil.
To make the egg ribbons: Slowly pour the beaten eggs into the hot broth. For thicker, fluffier ribbons, let the eggs set briefly before gently stirring. For thinner, feathery ribbons, gently stir the soup as you pour in the eggs.
To finish and serve: Add the rehydrated seaweed, sesame oil, and scallions. Gently stir, turn off the heat, and serve immediately.