Place 1 cut chicken on a plate.
Sprinkle 3 tablespoons of chopped green onion, 3 tablespoons of chopped cilantro, 1 teaspoon of minced garlic, 1 teaspoon of chopped onion, 1 teaspoon of salt, and 1/2 teaspoon of ground black pepper over the chicken.
Cover and marinate for 30 minutes.
Heat 1 tablespoon of vegetable oil in a deep pot over medium heat.
Stir in 1/4 cup of brown sugar and cook until it melts into a golden brown syrup.
Add the marinated chicken pieces and brown quickly while turning continuously.
Cover the pot and let it cook for 2 minutes.
Pour in 1 cup of water, 1/2 cup of canned coconut milk, and 1 teaspoon of red pepper flakes.
Replace the lid and cook over medium heat for 10 minutes.
Stir in 2 tablespoons of ketchup and 1 tablespoon of butter.
Continue cooking until chicken is fork-tender, 20 to 30 minutes.
Taste and season with additional salt and pepper if needed.
Serve chicken with the sauce in the pot as a gravy.