Combine Base Ingredients:
In a small saucepan, whisk together sugar, rice vinegar, ketchup, soy sauce, and 1 cup of water.
Heat the Sauce:
Place the saucepan over medium heat and bring the mixture to a simmer, stirring occasionally to dissolve the sugar.
Thicken the Sauce:
In a small bowl, mix the cornstarch with 1 tablespoon of water to create a slurry. Slowly pour the slurry into the saucepan, whisking constantly to avoid lumps.
Simmer and Finish:
Continue to cook the sauce for 1–2 minutes, stirring frequently, until it thickens to your desired consistency. Remove from heat and let it cool slightly.
Serve:
Use immediately as a dip or drizzle over your dish. Store leftovers in an airtight container in the refrigerator for up to 1 week.