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1
Combine soy sauce, mirin, sake, sugar, garlic, and freshly grated ginger in a bowl. Whisk until the sugar is fully dissolved. Add rice vinegar and sesame oil if using. Reserve half the sauce for basting and serving; use the remainder as the marinade. Submerge the chicken thighs in the marinade and refrigerate for a minimum of 30 minutes. The sauce is ready when the sugar is entirely dissolved and the liquid is uniform.
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2
Remove the chicken from the marinade and pat the surface dry with kitchen paper. Discard the used marinade. Place the chicken skin-side down in a heavy pan or set on a grill over medium-high heat. This is where the dish lives or dies: the heat must be high enough that the residual sugar caramelises against the protein, producing the characteristic glossy, dark glaze — too low and the sauce stews without colouring; too high and it scorches before the meat is cooked through. Cook until the underside is deep mahogany and pulls cleanly from the surface.
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3
Turn the chicken and continue cooking on the second side. In the final minutes of cooking, brush the reserved sauce over the surface in two or three successive applications, allowing each coat to set and tighten before applying the next. The glaze is done when it is lacquered, shining, and no longer wet-looking.
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4
Rest the chicken briefly off the heat. Bring any remaining reserved sauce to a brief simmer in a small pan and offer it alongside. The finished glaze should be glossy and cling to the meat without pooling.