Heat the broth with ginger and corn. Pour the broth into a medium pot, add the ginger slices and corn, and bring it to a gentle simmer over medium heat. Let it simmer 3 minutes so the ginger perfumes the broth.
Season before the eggs. Stir in the salt and white pepper now, so you stir as little as possible after the eggs go in.
Thicken with the slurry. Give the cornstarch mixture a fresh stir, then drizzle it into the simmering broth while stirring in a slow circle. Simmer 30 to 60 seconds, until the broth looks glossy and coats the back of a spoon lightly.
Set up the drizzle. Turn the heat down so the broth is barely simmering. Hold the bowl of beaten egg about 6 inches above the pot and let the egg fall in a thin thread, using chopsticks or a fork against the bowl edge to break the stream.
Move the broth, not the egg. Stir the broth slowly in one direction, about one circle every 2 seconds, so the moving broth stretches the egg into ribbons as it sets.
Rest, then finish. Turn off the heat and wait 30 seconds without stirring. Stir in the sesame oil, top with scallions, and serve right away.