In a small pot, combine soy sauce, 1/4 cup water, brown sugar, honey, rice vinegar, garlic, and ginger. Bring to a simmer over medium heat, stirring until sugar dissolves completely.
Mix cornstarch with 2 tablespoons cold water in a small bowl until no lumps remain.
Pour cornstarch slurry into simmering sauce while stirring constantly. Keep stirring for 1-2 minutes until thickened and it coats the back of a spoon. Remove from heat and set aside.
Pat chicken thighs dry with paper towels. Season both sides with salt and pepper.
Preheat Blackstone griddle for about 10 minutes to 400-425°F.
Drizzle 2 tablespoons vegetable oil onto griddle and spread evenly with a spatula.
Lay chicken thighs flat on griddle with space between each piece. Cook without touching for 5-6 minutes until a brown crust develops and they release easily from the griddle.
Flip chicken and cook the second side for another 5-6 minutes until internal temperature reaches 155-160°F.
Pour about half of the teriyaki sauce over the chicken. Use spatula to flip and coat chicken in the sauce for 2-3 minutes until sauce reduces and becomes sticky.
Continue adding more sauce as desired, flipping and coating until chicken reaches internal temperature of 165°F.
Remove chicken from griddle and let rest for a couple minutes.
Sprinkle with sesame seeds and sliced green onions if using. Serve over rice or with vegetables.