In a small bowl, whisk together balsamic vinegar, soy sauce, brown sugar, maple syrup, ketchup, Sriracha sauce, ginger, onion powder, 1/2 teaspoon pepper, and garlic powder. Chop green onions and set aside, separately.
Season chicken with salt and remaining pepper, then place in a slow cooker; pour the sauce over top. Cover and cook on high for 2 hours (or on low for 4 hours) until no longer pink.
Scoop sauce out from the slow cooker and place into a small saucepan, leaving the chicken to keep warm. Whisk cornstarch together with cold water until dissolved, then pour into sauce and heat over low. Stir continuously for 5 minutes until thickened.
Drizzle sauce over the chicken. Garnish with chopped green onions and sesame seeds.