In a small saucepan, combine the soy sauce, mirin, and sake.
Add the sugar, minced garlic, and ginger to the mixture.
Stir the ingredients over medium heat until the sugar dissolves.
Bring the mixture to a gentle simmer.
For a thicker sauce, mix cornstarch with a little water to create a slurry.
Add the cornstarch slurry to the saucepan if you desire a thicker consistency.
Keep stirring until the sauce thickens slightly, about 3-5 minutes.
Remove from heat and let cool slightly.
Transfer the sauce to a jar or bottle for storage.
Use immediately or refrigerate for up to two weeks.