Trim any excess fat from the chicken thighs. Place the chicken between two sheets of plastic wrap or parchment paper and lightly beat with a meat mallet until the meat is evenly thickened.
In a mixing bowl, combine gochujang, gochugaru, soy sauce, brown sugar, garlic, ginger paste, corn syrup, sweet rice wine (if using), sesame oil, and black pepper. Mix well.
Add the chicken pieces to the marinade, ensuring they are evenly coated. Cover and refrigerate for at least 1 hour (or overnight for deeper flavor).
Heat a skillet or grill pan over medium-high heat. Lightly oil the pan if needed. Add the marinated chicken and cook for 4-5 minutes on each side until fully cooked and caramelized. Internal temperature should reach 165°F (75°C).
Transfer the chicken to a serving plate. Garnish with thinly sliced perilla leaves and toasted sesame seeds. Serve with steamed rice, lettuce wraps, or pickled vegetables.
