Combine all of the ingredients for the marinade in a medium bowl or a Ziploc bag and mix thoroughly, making sure that the sugar has dissolved. Add the chicken thighs and marinate overnight. I find that overnight (or even up to 24 hours) marination gives the chicken optimal flavor and tenderness.
Preheat the grill to 300F - 325F at the grate level. You don't want the grill to be very hot to prevent burning.
Trim excess skin and fat from chicken thighs. Place the thighs in a medium bowl and add the marinade. Stir well to ensure proper coverage. Marinade for 24 hours or at least overnight.
Remove chicken thighs from the marinade and pat dry with paper towels. Some bits and pieces of herbs and chili pepper remaining on the skin are totally fine, but you want as much of the liquid and oil off as possible for better browning and to prevent flare-ups.
Grill chicken thighs over direct heat, flipping frequently, about every 2-3 minutes, until the chicken is cooked through and internal temperature has reached a minim of 165F.
Let the chicken rest for 3 minutes and serve.