Combine all the ingredients in a small pot and whisk until the sugar and cornstarch have dissolved. The sauce will have a slightly milky brown appearance. Heat the sauce over medium, while whisking, until it begins to simmer. Once it begins to simmer the cornstarch will thicken the sauce and it will turn from cloudy to shiny in appearance. Serve immediately. Serve the sweet and sour sauce immediately or refrigerate for up to one week.
