Preheat the oven to 400°F (200°C). Spray a 13x9x2-inch baking dish with nonstick cooking spray.
Place the 2 pounds of boneless skinless chicken breasts in a single layer in the prepared baking dish.
In a medium saucepan, whisk together 1/2 cup soy sauce, 1/2 cup granulated sugar, 1/4 cup rice vinegar, 1 minced garlic clove, 1/4 teaspoon ground ginger, and 1/4 teaspoon black pepper.
In a small bowl, mix 1 tablespoon cornstarch and 1 tablespoon water until smooth, then whisk this slurry into the saucepan.
Bring the sauce to a boil over medium-high heat, then reduce to low and simmer, whisking, until it begins to thicken.
Pour the teriyaki sauce evenly over the chicken in the baking dish.
Bake the chicken for 20 minutes, then spoon some of the sauce from the dish over the chicken pieces.
Return the dish to the oven and bake an additional 15 minutes, or until the chicken is cooked through and the juices run clear.
Garnish with chopped green onions and sesame seeds, then serve.
