Make the Sauce: In a medium saucepan, whisk together the tamari (or soy sauce), water, brown sugar replacement, allulose (or xylitol), sesame oil, minced garlic, fresh ginger, and hot pepper flakes.
Bring this mixture to a gentle boil over medium heat. Once boiling, sprinkle in the xanthan gum and whisk vigorously for a minute or two until the sauce slightly thickens. Remove it from the heat and let it cool down completely.
Prepare the Chicken: Cut the boneless, skinless chicken thighs into bite-sized pieces. In a bowl, toss the chicken with about two-thirds of the cooled sauce.
Cover the bowl and refrigerate for at least 2 hours to let the chicken marinate and soak up all those delicious flavors. Remember to set aside the remaining sauce for serving later!
Prepare Skewers: If you're using bamboo skewers, make sure to soak them in water for at least an hour before you start grilling. This prevents them from burning. You'll need around 6-8 large skewers, or 12 smaller ones if you're making appetizers.
Preheat your grill to medium-high heat and lightly grease the grates to stop the chicken from sticking. Carefully thread the marinated chicken pieces onto your soaked skewers.
Grill the Yakitori: Place the chicken skewers on the hot grill. Cook them for about 5-6 minutes on each side, brushing frequently with any leftover marinating liquid to keep them extra juicy. Grill until the chicken is beautifully browned and cooked through.
Once done, transfer the skewers to a serving platter. Sprinkle them generously with toasted sesame seeds and freshly chopped scallions. Serve immediately with the reserved sauce on the side for dipping!