Make the Sauce:
Combine the soy sauce, vinegar, mirin, sugar and sesame oil in a small bowl and stir until sugar dissolves. Just before you serve, stir in the scallions and shiso leaves.
Make the Chicken:
Combine the eggs, 2 tablespoons of the cornstarch, sake, soy sauce, sugar, garlic, ginger and salt in a large mixing bowl and stir until sugar dissolves. Add the chicken, toss well, and set aside to marinate for at least 15 minutes uncovered—or refrigerated (and covered) for up to a few hours.
Pour 2-3 inches of vegetable oil in a medium pot and set it over medium high heat until the oil reaches 350 degrees F on a deep fry thermometer.
Put the remaining 1 cup of cornstarch in a large mixing bowl. Lift half of the chicken out of the marinade, letting the excess drip off. Add it to the cornstarch and toss to thoroughly coat each piece. Do not shake off the excess cornstarch. Use tongs to add the chicken pieces one by one to the oil and fry, stirring and flipping the chicken occasionally, until it’s crispy and the visible meat is golden brown, about 6 minutes. Transfer them to paper towels or a cooling rack to drain and immediately sprinkle the Yuzo Kosho powder on top of the hot chicken. Repeat with the remaining chicken. Serve immediately with the dipping sauce alongside.