In a small bowl, whisk together cornstarch and water until smooth. Set aside.

In a medium saucepan over medium heat, combine brown sugar, soy sauce, garlic, rice vinegar, ginger, honey, sesame oil, and the cornstarch mixture. Stir frequently and bring to a gentle simmer. Cook for 2-3 minutes until the sauce thickens and turns a deep, glossy brown. Remove from heat and let cool completely (you can speed this up by placing the saucepan in the fridge).

Once the sauce is cool, transfer cubed chicken to a large resealable plastic bag. Pour half of the sauce over the chicken, seal the bag, and gently shake to coat evenly. Refrigerate for at least 30 minutes (up to 2 hours for deeper flavor). Reserve the remaining sauce for basting.

Preheat your grill to medium heat (about 350°F). If using wooden skewers, soak them in water for 30 minutes first. Thread the marinated chicken onto the skewers, leaving a small space between each piece.

Lightly oil the grill grates. Place the skewers on the grill and cook, turning every 3-4 minutes, until the chicken is cooked through and reaches an internal temperature of 165°F, about 20-30 minutes. During the last few turns, brush the skewers with the reserved teriyaki sauce for a glossy finish.

Remove skewers from the grill and let rest for 2 minutes. Garnish with sliced green onions and a sprinkle of sesame seeds if desired. Serve hot over steamed rice or with a side of grilled vegetables.