Make the marinade: In a bowl, whisk soy sauce, brown sugar, rice vinegar, sesame oil, neutral oil, garlic, ginger, smoked paprika, black pepper, and red pepper flakes (if using) until the sugar dissolves.
Prep the chicken: Pat chicken dry. If using breasts, pound them to an even 1/2–3/4 inch so they cook evenly. Add chicken to the marinade and toss to coat well.
Marinate: Cover and refrigerate for at least 30 minutes and up to 12 hours. Tip: 2–4 hours is a sweet spot for deep flavor without mushy texture.
Choose your cooking method: Grill: Preheat to medium-high and oil grates.
Grill thighs 5–6 minutes per side (breasts 4–5) until charred at the edges and cooked through.
Broil: Set oven rack 6 inches from heat. Broil on a foil-lined sheet 5–7 minutes per side, brushing with extra marinade halfway for shine and caramelization.
Skillet: Heat a large skillet or grill pan over medium-high with a little oil. Cook 5–6 minutes per side, letting the edges deeply brown.
Check doneness: Use an instant-read thermometer. Target 165°F in the thickest part.
Juices should run clear.
Rest the chicken: Transfer to a plate and rest 5 minutes. This keeps it juicy.
Optional quick sauce: Pour leftover marinade into a small saucepan. Bring to a boil for 1 minute.
Stir cornstarch and water to make a slurry, whisk in, and simmer 30–60 seconds until glossy. Never use raw marinade without boiling first.
Slice and serve: Slice chicken against the grain. Serve over rice with veggies, drizzle with sauce, and finish with scallions and sesame seeds.