In a small saucepan, combine Koikuchi Soy Sauce, Hon Mirin, Sake and Granulated Sugar over medium heat.
Bring the mixture to a gentle simmer, stirring so the sugar dissolves completely and the liquid is homogeneous.
Reduce the heat to low and let the sauce simmer uncovered for 5–10 minutes, stirring occasionally, until it reduces slightly and takes on a glossy, syrupy texture.
Taste and adjust: if it’s too salty, add a touch more mirin or a pinch of sugar; if too sweet, simmer a little longer to concentrate.
Remove from heat and let cool — the sauce will thicken further as it cools. For an ultra-smooth finish, strain through a fine-mesh sieve to remove any foam or undissolved particles.
Store in a sealed container in the refrigerator for up to 1–2 weeks. Rewarm gently before using and brush on grilled foods in the last minutes of cooking for a shiny glaze.
