Cut the chicken into 1-inch pieces and toss with the beaten egg. In a separate bowl, whisk together the flour, cornstarch, salt, garlic powder, and onion powder.
Preheat the air fryer to 390°F for 2–3 minutes.
Let the excess egg drip off each piece of chicken, then dredge it in the flour mixture, shaking off the extra.
Spray the air fryer basket, add the chicken in a single layer without touching, and spray the tops generously. Air fry for 8 minutes, flipping and spraying again halfway, until golden and 165°F inside. Work in batches if needed.
While the chicken cooks, whisk together the chicken stock, brown sugar, soy sauce, rice vinegar, hoisin sauce, and cornstarch.
Heat the vegetable oil in a large skillet over medium-high heat. Add the garlic, ginger, and chili flakes and stir for 15 seconds. Add the sauce and cook for 30 seconds to 1 minute, until glossy and thick.
Add the hot chicken, toss to coat, and garnish with scallions and sesame seeds. Serve immediately.