Add the garlic to the pan and cook for 30 seconds. Add the soy sauce, 2 tablespoons toasted sesame oil, water and brown sugar to the pan and bring to a simmer.
Mix the 4 teaspoons of cornstarch with 2 tablespoon of cold water. Add the cornstarch to the sauce and bring to a boil; boil for 30-60 seconds until just thickened.
Add the chicken back to the pan and toss to coat with the sauce. Serve over rice if desired.