Toss the chicken pieces with cornstarch, salt, and pepper until every piece is evenly coated. Spray lightly with cooking spray.
Preheat the air fryer to 390F. Arrange the chicken in a single layer without overcrowding and air fry for 12-14 minutes, shaking the basket every 4 minutes, until golden and crispy on all sides.
While the chicken cooks, combine the hot sauce, brown sugar, apple cider vinegar, soy sauce, garlic powder, and red pepper flakes in a small saucepan. Bring to a simmer over medium heat. Add the cornstarch slurry and stir until the sauce thickens, about 1-2 minutes.
Transfer the crispy chicken bites to a large bowl. Pour the hot firecracker sauce over the chicken and toss until every piece is well coated.
Serve immediately, garnished with sliced green onion. These are perfect as an appetizer or served over steamed rice.