Combine Ingredients: In a small saucepan, whisk together soy sauce, sugar, mirin, and sake (if using) until the sugar dissolves.
Simmer: Bring the mixture to a gentle boil over medium heat, then reduce to low heat.
Reduce: Simmer for 5–8 minutes, stirring occasionally, until the sauce thickens enough to coat the back of a spoon. (Avoid overcooking, as it will thicken further upon cooling.)
Cool & Store: Remove from heat and let cool. Transfer to a sterilized jar or bottle. Refrigerate for up to 2 weeks.
