In a bowl, whisk together the soy sauce, brown sugar, rice vinegar, sesame oil, minced garlic, and grated ginger until well combined to create the teriyaki sauce.
Place the chicken breasts in a large skillet over medium heat. Cook for about 5 minutes on each side until browned and cooked through, reaching an internal temperature of 165°F (75°C).
Pour the teriyaki sauce over the cooked chicken in the skillet. Bring to a simmer and let it cook for about 3-4 minutes, allowing the flavors to meld.
In a small bowl, mix the cornstarch with water to create a slurry. Pour this into the skillet and stir until the sauce thickens, about 2 minutes.
Remove the skillet from heat and let it sit for a minute. Slice the chicken and serve on a plate drizzled with extra teriyaki sauce, then sprinkle with sesame seeds and chopped green onions.
Pair with steamed rice and your favorite vegetables for a complete meal.