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1
Combine soy sauce, mirin, and sake in a small saucepan in the ratio 4 : 3 : 2. If sugar is used, add it now. Stir briefly over low heat until any sugar dissolves — the liquid should be uniform in colour with no granules visible at the base of the pan.
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2
Raise the heat to a moderate simmer. For a basting glaze or coating sauce, reduce the liquid until it thickens to a syrup consistency: a spoon drawn across the surface leaves a brief trail before the sauce closes over it. For a marinade or table condiment, remove from heat before reduction begins and allow to cool. This is the step on which the dish lives or dies — reduction beyond syrup produces a sauce that scorches on contact with the grill, while insufficient reduction yields a glaze too thin to adhere.
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3
Where sugar has been included, monitor closely throughout reduction: caramelisation begins rapidly and the sauce can cross from glossy to bitter without warning. Mirin alone carries sufficient natural sugars; when precise heat control is not available, omit added sugar entirely. The finished sauce should coat the back of a spoon with a clean, even film.
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4
For the basting application, allow the reduced sauce to cool slightly before the first brush coat is applied to the protein; successive coats build the characteristic lacquered surface. For table use, decant into a small vessel and serve at room temperature. The sauce prepared in advance and stored will thicken further on standing.