In a shallow dish, combine cornstarch, panko breadcrumbs, and salt. Set aside.

Cut chicken breast into 1-inch pieces. Dip each piece of chicken into the beaten egg, then dredge thoroughly in the breadcrumb mixture, ensuring it's fully coated. Gently shake off any excess.

Lightly spray the air fryer basket with cooking spray. Arrange chicken pieces in a single layer, ensuring not to overcrowd. Spray the tops of the chicken with cooking spray.

Air fry at 375°F (190°C) for 10-12 minutes, flipping halfway through, until golden brown and cooked through (internal temperature reaches 165°F/74°C). Work in batches if necessary.

While chicken cooks, prepare the teriyaki sauce: In a small saucepan, combine soy sauce, water, brown sugar, grated ginger, and minced garlic. Bring to a simmer over medium heat, then reduce heat to low and cook for 5-7 minutes, stirring occasionally, until slightly thickened.

Once chicken is cooked, transfer to a large bowl. Pour the teriyaki sauce over the chicken and toss gently to coat all pieces evenly. Serve immediately.