Combine the ½ cup water, vinegar, brown sugar, ketchup and soy sauce in a small sauce pan. Bring to a boil over medium high heat.
Meanwhile, combine cornstarch and 2 tablespoons of cold water in a small bowl until well mixed.
Once the sweet and sour sauce mix has come to a boil, stir in the cornstarch slurry and stir constantly, for about a minute, until the sauce has thickened.
Remove from heat and use immediately or store in a jar or air-tight container for up to 2 weeks. Reheat and add extra water, as needed, to get it to the right consistency.
