Pat the chicken breast dry and cut into bite-sized pieces. Season with salt, black pepper, and onion powder. Add the egg, then mix until the chicken is evenly coated.
In a separate bowl, whisk together cornstarch, flour, and baking powder.
Dip each piece of chicken into the dry mixture, pressing the coating onto the chicken to create a thick, crispy crust.
Lightly spray the air fryer basket with oil and arrange the chicken pieces in a single layer, making sure they aren't touching. Lightly spray the tops with oil.
Air fry at 400°F for 12–15 minutes, flipping halfway through and spraying with a little more oil. Cook until golden and the chicken reaches 165°F internally.
While the chicken cooks, add gochujang, honey, soy sauce, rice vinegar, garlic, and water to a small saucepan. Simmer over medium-low heat for 2–3 minutes, stirring until smooth and slightly thickened.
Right before serving, toss the crispy chicken with the gochujang sauce until evenly coated.
Sprinkle with sesame seeds and sliced green onions.