Step 1

Mix ketchup, soy sauce, vinegar, and sugar in a bowl.

Step 2

Whisk together the cornstarch and the water in a small bowl.

Step 3

Pour the cornstarch mixture into the sauce bowl and stir until well combined.

Step 4

Cook the mixture in a saucepan over high heat until it just begins to boil, then reduce the heat to medium-low and cook until it thickens, stirring occasionally. Remove the saucepan from the heat and let the sauce cool.