In a large bowl, combine brown sugar, soy sauce, rice vinegar, sesame oil, gochujang paste, ginger, garlic, green onions, and sesame seeds. Whisk thoroughly until sugar dissolves.

Reserve 1/4 cup of the marinade for basting later (store in refrigerator until needed).

Place chicken pieces in a large ziplock bag or shallow dish and pour the remaining marinade over them. Ensure all pieces are well coated.

Seal and refrigerate for at least 2 hours, or preferably overnight. Turn occasionally to ensure even marinating.

When ready to cook, remove chicken from refrigerator and let sit at room temperature for 20 minutes.

Preheat grill to medium-high heat (around 375-400°F) or oven to 425°F.

Brush grill grates with vegetable oil to prevent sticking.

Remove chicken from marinade, allowing excess to drip off.

For grilling: Place chicken on grill, skin side down if using skin-on pieces. Grill for 5-7 minutes per side until char marks appear, basting occasionally with the reserved marinade.

For oven baking: Place chicken on a lined baking sheet and bake for 25-30 minutes, basting halfway through.

Check that chicken reaches an internal temperature of 165°F.

Transfer to a serving plate and let rest for 5 minutes before serving.

Garnish with additional sliced green onions and sesame seeds.