Add the sugar, water, white vinegar, cornstarch, and ketchup to a medium saucepan, then whisk until the mixture looks evenly blended. The liquid will start out thin and cloudy, and you should not see dry pockets of cornstarch sitting on the bottom.

Set the pan over medium-high heat and cook, whisking continuously, until the sauce comes to a boil and thickens, about 3 minutes. Watch for the color to shift from cloudy to glossy translucent red, and feel for a little drag on the whisk as the glaze forms.

Take the pan off the heat and let the sauce cool completely before transferring it to an airtight container and refrigerating it. The sauce will continue to set a bit as it cools, so it should look slightly looser in the pan than it will later.