1/4 cup soy sauce
1/4 cup Marsala wine
1 teaspoon sugar
1 tablespoon rice vinegar
1 tablespoon mirin (sweet Japanese cooking wine) or sweet wine
2 teaspoons cornstarch
Bring the soy sauce, Marsala, sugar, rice vinegar, and mirin to a boil in a small saucepan. Simmer 3 minutes.
Combine the cornstarch with 2 tablespoons of cold water to form a slurry. Thicken the sauce by stirring in the cornstarch slurry, bit by bit, until the sauce has a smooth consistency and coats the back of a spoon.