Combine Ingredients:
In a medium saucepan, combine soy sauce, 1/4 cup water, brown sugar, rice vinegar, mirin (if using), minced garlic, and grated ginger.
Heat the Sauce:
Place the saucepan over medium heat and whisk until the sugar dissolves and the mixture is well combined.
Thicken the Sauce:
In a small bowl, mix the cornstarch with 2 tablespoons of water to create a slurry. Slowly pour the slurry into the saucepan while whisking.
Simmer:
Reduce the heat to low and let the sauce simmer for 2–3 minutes, stirring frequently, until it thickens to your desired consistency.
Add Sesame Oil:
Stir in the sesame oil for an extra layer of flavor, if desired.
Cool and Store:
Remove the sauce from the heat and let it cool slightly. Use immediately or store in an airtight container in the refrigerator for up to 1 week.