In a large bowl, combine soy sauce, brown sugar, vegetable oil, minced garlic, minced ginger, rice vinegar, and black pepper. Whisk until the sugar is dissolved.
Add the chicken thighs to the marinade, ensuring they are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
In a large pot, heat the chicken broth over medium heat. Once it begins to simmer, reduce the heat to low and keep warm.
In a large skillet over medium-high heat, add the marinated chicken thighs, skin side down. Cook for about 5-7 minutes until the skin is golden brown and crispy.
Flip the chicken thighs and pour the remaining marinade over them. Reduce the heat to medium, cover, and cook for an additional 15-20 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F.
Remove the chicken from the skillet and let it rest for a few minutes. Meanwhile, increase the heat to high and let the sauce simmer for a few minutes until it thickens slightly.
Serve the chicken thighs over a bowl of warm chicken broth, drizzled with the thickened sauce. Garnish with sliced green onions and sesame seeds.