Debone chicken thighs, cut into 1½-inch pieces with skin on, and toss with 1½ tablespoons sake and cornstarch.
Mix soy sauce, sugar, and 1½ tablespoons sake in a bowl; microwave 30 seconds to dissolve the sugar. Strain ginger juice into the sauce.
Heat oil in a skillet. Cook chicken skin-side down until edges turn white, 6 to 8 minutes. Flip and cook 1 to 2 minutes more.
Transfer chicken to a plate, discard fat, and wipe the pan clean.
Return chicken to the skillet with the sauce. Cook until the glaze thickens, 1 to 2 minutes.
Drizzle with 2 tablespoons of glaze; serve extra glaze on the side.