In a large bowl, whisk together the soy sauce, brown sugar, rice vinegar, sesame oil, garlic, ginger, and black pepper until the sugar is dissolved.
Add the chicken thighs to the marinade, ensuring they are well coated. Cover and refrigerate for at least 1 hour, or up to overnight for more flavor.
Preheat your oven to 375°F (190°C).
Place the marinated chicken thighs in a baking dish, skin side up, and pour the remaining marinade over the top.
Bake in the preheated oven for 35-40 minutes, or until the chicken is cooked through and the skin is crispy.
For extra crispiness, broil the chicken for an additional 2-3 minutes at the end of cooking.
Remove from the oven and let rest for 5 minutes. Garnish with sliced green onions and sesame seeds before serving.