Gather all ingredients and measure them precisely for the perfect balance of flavors.

Combine the soy sauce, sugar, mirin, and sake (if using) in a small saucepan.

Place the saucepan over medium heat and bring the mixture to a gentle simmer.

Reduce the heat to low and continue simmering for about 10 minutes, stirring occasionally, until the sauce has thickened to a syrup-like consistency that coats the back of a spoon.

Remove from heat and let cool completely. The sauce will continue to thicken as it cools.

Transfer to an airtight container or bottle for storage.