In a bowl, whisk together soy sauce, honey, rice vinegar, water, garlic, ginger, sesame oil, brown sugar (if using), and red pepper flakes.
Place the chicken directly into the crockpot in a single layer.
Pour the prepared teriyaki sauce over the chicken, ensuring it is well coated.
Cover and cook on LOW for 4–6 hours or HIGH for 2–3 hours, until chicken is fully cooked and tender (internal temperature of 165°F / 74°C).
Remove chicken and shred or slice as desired.
To thicken the sauce, mix cornstarch and cold water, stir into the crockpot sauce, and cook on HIGH for 5–10 minutes until glossy.
Return chicken to the crockpot, toss to coat, and serve warm over rice or noodles.