Put the cut chicken in a bowl and stir in 1 tablespoon soy sauce, beaten egg, salt, and pepper. Mix until pieces are evenly coated and let sit while you prepare the dry dredge (5–10 minutes).
In a shallow dish combine the cornstarch, flour, and baking powder. Toss the marinated chicken in the dry mixture a few pieces at a time so each piece is well coated; shake off excess and place on a wire rack. Let the coated chicken rest 10 minutes — this helps the coating adhere and crisp up when fried.
While the chicken rests, whisk together the sauce ingredients: brown sugar, 3 tablespoons soy sauce, rice vinegar, sesame oil, grated ginger and minced garlic in a small bowl. Mix the cornstarch slurry (1 tablespoon cornstarch with 3 tablespoons water) and set both aside.
Heat oil in a deep skillet or Dutch oven to 350°F (use a thermometer if available) — you want enough oil to allow the chicken to float while frying. Fry the chicken in 2–3 small batches so the oil temperature recovers quickly. Cook each batch 3–4 minutes until golden and crisp, turning once.
Remove fried pieces with a slotted spoon and drain briefly on a wire rack set over a baking sheet (avoid paper towels so crispness is maintained). Keep cooked pieces warm in a 200°F oven if desired while finishing remaining batches.
Pour the prepared sauce into a medium skillet over medium heat. Bring to a gentle simmer, then whisk in the cornstarch slurry. Cook, stirring, until the sauce thickens to a glossy glaze (about 1 minute). Taste and adjust: add a pinch of salt or a splash of rice vinegar if you prefer tangier notes.
Add the warm fried chicken to the skillet with the glaze. Toss quickly and gently over low heat so each piece gets an even, sticky coating without softening the crust — 30–60 seconds is enough.
Transfer to a serving platter, sprinkle with toasted sesame seeds and sliced green onions, and serve immediately with steamed rice or stir-fried vegetables.