In a small saucepan, combine the mirin, soy sauce, and sugar.

Place the pan over medium-high heat and bring the mixture to a gentle boil. Once boiling, reduce the heat to medium-low and allow it to simmer for 5 to 10 minutes, stirring occasionally, until the sauce has thickened and reduced.

In a separate bowl, whisk together the cornstarch and water to create a smooth slurry.

Slowly pour the slurry into the simmering sauce, stirring continuously to ensure it fully incorporates.

Let the sauce cook for an additional 2 to 3 minutes, or until it reaches a thickness that can coat the back of a spoon. If you prefer a denser sauce, you may continue to reduce it further.

Remove the saucepan from the heat and allow the sauce to cool completely before serving.