In a mixing bowl, combine shoyu, mirin, sake, sugar, minced ginger, and garlic to create a fragrant marinade. Stir well until the sugar is dissolved, and set aside a portion of this sauce for glazing later.

Place the boneless skinless chicken thighs in a large resealable plastic bag or a glass dish. Pour the prepared teriyaki sauce over the chicken, ensuring it's fully coated. Seal the bag or cover the dish, then refrigerate for at least 2 hours.

Preheat your air fryer to 380°F (193°C). Lightly spray the basket with cooking oil to prevent sticking.

In a small saucepan, pour in the reserved teriyaki sauce for glazing. Whisk in a mixture of cornstarch and water until smooth, then cook for about 2-3 minutes until the sauce thickens.

Carefully arrange the marinated chicken thighs in the preheated air fryer basket. Air fry for 6 minutes, then flip the chicken pieces over. Brush generously with the thickened glaze, and continue cooking for an additional 5-6 minutes, or until the internal temperature reaches 165°F (74°C).

Remove the chicken from the air fryer and let it rest for a minute. Slice for serving, and add extra glaze if desired.