Add the mirin, sake, soy sauce, and brown sugar to a saucepan over medium heat. Bring to a gentle boil, stirring to dissolve the sugar, about 4–5 minutes. Keep the pan uncovered.
Reduce the heat to medium-low and simmer until the sauce thickens slightly, about 10 minutes, stirring occasionally. Remove from the heat and let cool. The sauce should be glossy and lightly viscous, not thick or syrupy. Measure out about ⅔ cup for the chicken.
Prepare the chicken
Preheat the air fryer to 400°F (204°C) for 5 minutes.
Dice the chicken into 1 to 1½-inch pieces. Add the chicken, salt, garlic powder, and starch to a large zip-top bag. Seal and shake until the chicken is evenly coated.
Air fry
Lightly spray the air fryer basket with oil. Arrange the chicken in a single layer with space between the pieces. Spray the tops lightly with oil.
Air fry for 8 minutes, then open the basket, shake or flip the chicken, spray lightly with oil again, and continue cooking for 2–4 minutes, until cooked through and lightly crisp on the outside.
Finish and serve
Transfer the hot chicken to a serving bowl. Pour the teriyaki sauce over the chicken and toss to coat evenly. Garnish with spring onions and sesame seeds. Serve right away.
Notes
Teriyaki sauce consistency: This sauce is meant to be glossy and lightly viscous, not thick or syrupy. If you prefer it thicker, simmer 2–3 minutes longer, or stir in a slurry of 2 teaspoons starch + 2 tablespoons water while simmering.
Salt level: Because the teriyaki sauce is already salty-sweet, the chicken is lightly seasoned on purpose. Don’t increase the salt — the sauce will finish the seasoning.
Batch cooking: For a 6-quart air fryer, cook the chicken in two batches. Spread the pieces in a single layer with a little space between them so they crisp instead of steaming.
Low-carb coating option: You can use unflavored whey protein isolate instead of starch. The chicken will turn a deeper golden color, but it may stick more to the basket compared to starch.
Make-ahead & storage:
Sauce: Let cool completely, then store in a sealed jar in the fridge for up to 3 weeks. It will thicken slightly as it chills.
Chicken prep: You can slice and season the chicken (without starch) 2–3 days ahead. Coat with starch right before air frying.
Leftovers: Store chicken and sauce separately. Reheat chicken in the air fryer at 370°F (188°C) for 5 minutes. The sauce does not need reheating.