Combine ingredients: In a small saucepan, mix soy sauce, mirin, sugar, and sake (if using). Stir until sugar dissolves.
Simmer: Place over medium heat and bring to a low simmer. Stir occasionally to prevent burning.
Thicken (optional): If you want a thicker sauce, mix cornstarch with 1 teaspoon water to make a slurry. Slowly whisk into the simmering sauce and cook 1–2 minutes until thickened.
Taste & adjust: Check sweetness and saltiness. Add a little extra sugar or mirin if needed.
Cool & store: Remove from heat, let it cool slightly, and use immediately or store in an airtight jar in the fridge for up to 2 weeks.
