Spray a 9x13-inch baking pan with nonstick cooking spray and place chicken breasts in the pan.
In a saucepan over medium heat, whisk together garlic, soy sauce, water, vinegar, brown sugar and cornstarch for a few minutes until the sauce starts to thicken.
Brush chicken with the sauce. Turn pieces over, and brush again.
Bake for 30 minutes. Remove from the oven, turn chicken over and bake for another 20-30 minutes, or until chicken is no longer pink.