If using wooden skewers, soak them in water for at least 30 minutes before grilling.
Make the tare sauce: combine soy sauce, mirin, and sugar in a small saucepan and bring to a boil, stirring until the sugar dissolves; remove from heat and transfer to a small bowl to cool slightly.
Thread the chicken onto skewers, about 3 pieces per skewer, separating pieces with sections of the white scallion.
Preheat a grill or grill pan to medium-high heat.
Lightly sprinkle sea salt over the chicken on both sides, then place the skewers on the hot grill; turn frequently to cook and char evenly, about 3–4 minutes total depending on thickness.
When the chicken is almost cooked through, dip each skewer into the tare sauce and return to the grill; cook about 30–40 seconds per side, basting so the sauce glazes the chicken.
Remove skewers from the grill and serve immediately.
