1/3 cup rice wine vinegar
4 tablespoons brown sugar
1 tablespoon ketchup
1 teaspoon soy sauce
2 teaspoons cornstarch mixed with 4 teaspoons water
Mix the vinegar, brown sugar, ketchup, and soy sauce together and bring to a simmer in a small saucepan.
Mix together the cornstarch and water, add to the other ingredients, bring back to a simmer and stir to thicken.