In a small bowl, whisk the cornstarch with the water. In a small saucepan, whisk the soy sauce with the sake, mirin and sugar. Bring to a simmer over moderate heat, stirring until the sugar is dissolved, about 2 minutes. Whisk in the cornstarch mixture and simmer until the sauce is slightly thickened, about 2 minutes longer. Transfer the glaze to a heatproof jar and let cool completely.