In a mixing bowl, combine the chicken pieces with salt, pepper, and egg. Allow it to marinate for about 15 minutes.

In a separate bowl, whisk together cornstarch and flour. Dredge each piece of marinated chicken in this mixture, ensuring each piece is well-coated.

Preheat your air fryer to 400°F (200°C). Place the coated chicken in the fryer basket in a single layer. Air fry for 10-12 minutes, shaking halfway through, until golden brown and cooked to an internal temperature of 165°F (75°C).

In a saucepan, mix chicken broth, brown sugar, hoisin sauce, soy sauce, garlic, ginger, rice vinegar, sesame oil, and chili flakes. Simmer over medium heat until the sugar dissolves. Mix in the tablespoon of cornstarch for thickness.

Once the chicken is crispy, pour the sauce over it in a bowl, tossing to coat well. Serve hot, garnished with green onions and sesame seeds.