Marinade chicken
Combine garlic, ginger, soy sauce (shoyu), mirin, and sake in a bowl or zip-top bag.
Debone the chicken thighs and trim any excess skin.
Marinate the chicken in the soy sauce mixture for 15 minutes only (longer and it becomes too salty).
Grill chicken
Start a charcoal grill and set up a two-zone fire.
Grill the chicken skin-side down first. Move pieces to a cooler zone if flare-ups occur; a little char is good.
Grill until 155°F internal, then rest 5 minutes.
Finish in sauce
Cut the rested chicken into bite-sized pieces, skin-side down on the board so the skin stays attached to each piece.
Heat a large nonstick skillet over medium heat.
Mix corn starch with 1 tbsp cool water to make a slurry.
Add chicken pieces and Tare No Teriyaki to the skillet.
When the sauce bubbles, stir in the slurry and toss until the sauce thickens and coats the chicken; turn off heat immediately once thickened.
Transfer chicken and sauce to a serving dish.