In a small bowl, whisk together the brown sugar, ketchup, soy sauce, chicken broth, ginger, and minced garlic until smooth.
Reserve 1 cup of the mixture in a covered container and refrigerate for basting.
Place the remaining marinade and the chicken thighs into a large resealable bag or container, remove excess air, and refrigerate overnight to marinate.
When ready to cook, discard the marinade that held the raw chicken.
Preheat a grill or grill pan to medium-high heat. Grill the thighs 5–7 minutes per side, brushing occasionally with the reserved marinade, until the internal temperature reaches 165°F.
Remove chicken from the grill, let rest a few minutes, then serve warm.