Combine soy sauce, honey, hoisin sauce, rice vinegar, ginger, garlic, sesame oil, onion powder, and red pepper flakes.
Place chicken breasts in the bottom of a slow cooker and pour the sauce over top. Cook on low for 4 to 5 hours or until the chicken reaches 165°F in the thickest part.
Remove the chicken from the slow cooker and place it in a bowl. Use two forks to shred it, discarding the bones. Lightly cover to keep warm and set aside.
In a small bowl, combine 3 tablespoons of water and cornstarch. Transfer the liquid from the slow cooker to a medium saucepan and bring to a boil over medium-high heat. While whisking, drizzle in the cornstarch mixture a little at a time to reach the desired consistency; you may not need all of the mixture. Let boil 1 minute.
Pour the sauce over shredded chicken and stir to coat. Garnish with green onion and sesame seeds if desired.
Serve over rice or noodles.
