In a medium bowl, whisk together the brown sugar, ketchup, soy sauce, and chicken broth until smooth.

Stir in the minced ginger, garlic, rice vinegar, and sesame oil.

Set aside 1 cup of this marinade to use for basting later, and store it in the fridge.

Place the remaining marinade in a large freezer bag with the chicken thighs. Seal the bag and massage it gently to ensure the chicken is evenly coated.

Let the chicken marinate in the fridge overnight, or at least for 8 hours.

Preheat your grill and lightly oil the grates.

Remove the chicken from the marinade bag, discarding the used marinade.

Grill the chicken for about 3 minutes on one side. Brush some reserved marinade over the cooked side.

Flip the chicken and grill for another 3 minutes, again basting with the marinade.

Repeat this process until the chicken is fully cooked, which should take about 10 minutes total. Ensure the internal temperature reaches 165°F.