Combine coconut sugar, mirin, tamari (or soy sauce or coconut aminos) and vinegar in a small saucepan.
Before heating, transfer ¼ cup of the mixture to a resealable plastic bag or marinating container.
Slice chicken thighs lengthwise, add to the bag and massage to coat. Chill in the refrigerator for 2 hours or more.
Simmer the remaining marinade over low heat until reduced by half. It will have the consistency of syrup. Remove from heat.
Heat one side of a gas grill to medium-high and the other to low. (If using coals, bank them on one side). Oil the grates. Thread chicken on to 4 skewers, accordion style.
Grill over direct heat until nicely browned, 3-5 minutes. Remove to cooler side of grill and continue to cook, basting and turning every minute or so until cooked through, about 7 - 10 minutes depending on the temperature of the grill. Use an instant-read thermometer to make sure the internal temperature of the chicken is at least 165°F.
Remove to a serving platter, top with sesame-lemon sprinkle and serve.
You can make the marinade ahead if you'd like. Cool and chill, then bring to room temperature before using. See the recipe post for freezing instructions.