Begin by cutting the chicken into bite-sized pieces (about 1-inch cubes).
In a medium bowl, whisk together the soy sauce, brown sugar, rice vinegar, sesame oil, minced garlic, and grated ginger until well combined. Set the teriyaki sauce aside.
Heat the large skillet over medium heat. Add the chicken pieces and cook until golden brown and cooked through, about 5-7 minutes, stirring occasionally.
Once the chicken is cooked, pour the teriyaki sauce over the chicken in the skillet. Stir to coat the chicken evenly.
In a small bowl, mix the cornstarch with water to create a slurry. Add this mixture to the skillet and cook for an additional 2-3 minutes, stirring constantly, until the sauce thickens.
Remove from heat and serve immediately over cooked rice or with stir-fried vegetables. Garnish with chopped green onions.
Optionally, add vegetables like bell peppers, broccoli, or snap peas to the skillet during the last few minutes of cooking for additional flavor and nutrition.