Combine soy sauce, water, brown sugar, minced garlic, and grated ginger in a saucepan
Heat over medium heat until the sugar dissolves
Stir in honey or mirin if desired
Mix cornstarch with cold water in a small bowl
Add the cornstarch slurry to the saucepan
Simmer and stir until the sauce thickens
Remove from heat
Let cool before using
Store in an airtight container in the refrigerator