Reserve 2 Tbsp chicken broth. Bring remaining broth to a boil in a small saucepan.
Reserve some sliced green onion for garnish. Add remaining green onion, ginger, garlic, vinegar and mirin to the broth in the saucepan. Reduce heat to a bare simmer. Simmer, covered, for 20 minutes to extract flavors.
Strain broth to remove solids. Return broth to the saucepan over medium heat.
Make a slurry of the cornstarch and reserved chicken broth. Add the slurry to the simmering broth, stirring until it becomes clear and thickens slightly. Continue to simmer for 5 minutes.
In a small bowl, beat egg with a fork. Remove broth from the heat and stir. Drizzle beaten egg into hot soup in a thin stream, moving the stream across the surface of the soup to make streamers, rather than clumps. Still off-heat, cover the pot for 1 minute while preparing optional garnishes.
In the serving bowl, gently heat cooked chicken and corn kernels (if using) in the microwave for 20 – 30 seconds, if required.
Gently pour soup over chicken and corn. Garnish with reserved green onions.