Special equipment: cheesecloth
Pour 10 cups water into a big stock pot and bring to a boil over high heat. While it's coming to temp, throw in the leek, carrot, and hunk of ginger. Place the coriander and peppercorns into a small square of cheesecloth. Tie with kitchen string and add to the pot.
Skin the chicken breasts, remove any excess fat, and throw the chicken breasts into the pot. Allow the whole lot to come to a boil, and then turn it down and simmer, covered, until the chicken is cooked and tender, 20 to 30 minutes.
Remove the chicken and allow to cool. Strain the stock and discard the leeks, carrots, ginger, and cheesecloth sachet.
Return the stock to medium heat. Add the creamed corn, bouillon cubes, chopped ginger, scallions, and sesame oil. Keep the soup at a gentle simmer while you remove the chicken from the bone and shred finely using a fork. (Or you can just slice it really thinly.)
Turn up the heat to high and bring the soup to a boil. In a small bowl or measuring cup, mix the cornstarch with the water until smooth. Add half to the soup whilst stirring. Allow to boil until it's thickened to the soup consistency of your liking. (If it doesn't thicken, add the remaining slurry.)
Turn the heat back down to medium. In another small bowl, beat the egg whites with a little water. Stir the soup in circles as you add the egg whites in a steady stream; it should form pretty white wisps on the surface of the soup.
Add the shredded chicken and rice vinegar. Cook for 5 minutes, allowing everything to warm through. Taste for seasoning. Serve with a splash of soy sauce and some of the reserved scallion greens.