In a medium-sized bowl, mix together the sweet chili sauce, soy sauce, rice vinegar, sesame oil, minced garlic, grated ginger, black pepper, and red pepper flakes if desired. Stir well until combined.
Cut the chicken into bite-sized pieces and add to the marinade, ensuring each piece is coated. Cover and marinate in the refrigerator for at least 30 minutes.
Preheat your air fryer to 380°F (193°C).
Remove the chicken from the marinade and let excess liquid drip off. Coat the chicken pieces with cornstarch.
Place the coated chicken in a single layer in the air fryer basket. Cook for 12-15 minutes, shaking or flipping halfway through.
Check the internal temperature of the chicken to ensure it reaches at least 165°F (74°C).
Remove the chicken from the air fryer and serve, garnishing with green onions and sesame seeds.