In a small saucepan, add the sugar, vinegar, and ⅔ cup of water. Place over medium heat. Whisk constantly until the sugar completely dissolves.
Once the sugar is melted, add the ketchup to the saucepan and whisk until well combined and smooth.
In a small bowl, mix together the cornstarch and 2 tablespoons of water to form a slurry.
Bring the mixture to a gentle boil. Slowly stream in the cornstarch slurry while whisking continually. Keep whisking until the sauce thickens, about 30 seconds.
Remove the saucepan from heat and let the sauce cool before using, or serve immediately.
Notes
Adjust sweetness by adding more sugar or ketchup.
For richer flavor, add low-sodium soy sauce.
Store leftover sauce in an airtight container in the fridge for up to a week.
Avoid rushing the heating process to prevent graininess.
Customize flavor with garlic powder or crushed red pepper.