Preheat the oven to 400°F. Line a rimmed baking sheet with foil and set a wire rack on top.
Whisk soy sauce, ketchup, honey, brown sugar, vinegar, water, oil, garlic, onion powder, ginger, black pepper, and red pepper flakes in a bowl.
Pat chicken dry. Toss with about half of the sauce to coat well.
Arrange chicken on the rack, skin side up. Reserve the remaining sauce for basting.
Bake for 20 minutes. Baste the tops with some of the reserved sauce.
Flip the pieces and bake 15 minutes more. Brush with more sauce.
Continue baking 10–15 minutes until sticky, deeply browned, and at least 175°F inside. Broil 1–2 minutes if you want extra char.
Rest 5 minutes. Toss with pan juices, then finish with sesame seeds and green onions.