In a mixing bowl, whisk together soy sauce, brown sugar, water, honey, garlic, ginger, sesame oil, pineapple juice, apple cider vinegar, black pepper, and red pepper flakes until well combined.
Place the chicken breasts in a large ziplock bag and pour the marinade over the chicken. Seal the bag and ensure all pieces are well coated. Marinate for at least 2 hours, or overnight for best flavor.
Preheat the grill to medium-high heat. Remove the chicken from the marinade and grill for 5-7 minutes on each side or until the internal temperature reaches 165°F (74°C).
While the chicken is grilling, pour the remaining marinade into a small saucepan. Mix cornstarch with 2 tablespoons of water and stir into the marinade. Bring to a boil, then reduce heat and simmer until the sauce thickens.
Brush the thickened sauce over the grilled chicken. Garnish with green onions and sesame seeds.