1. In a bowl, toss chicken with cornstarch. Set aside until ready to cook.
2. Prepare the sauce: In a small saucepan, combine soy sauce, water, brown sugar, sesame oil, rice vinegar, garlic, and ginger. Bring to a boil. Stir in cornstarch mixture; return to a boil until sauce thickens. Remove from heat; set aside.
3. Heat a sauté pan over medium-high heat; coat with nonstick cooking spray. When hot, add chicken; cook until golden on all sides, about two to three minutes per side. Add sauce and toss, letting the sauce thicken to coat the chicken.
4. Transfer chicken to a serving dish or plates; sprinkle with sesame seeds and scallions.