Combine the ingredients in a small saucepan. Bring to a boil over medium-high heat.
1 cup water, 1 cup rice wine vinegar, 1/4 cup granulated sugar, 1/4 cup brown sugar, 2 teaspoons ketchup, 1 - 2 teaspoons crushed red pepper flakes, 2 garlic cloves, 1 tablespoon ginger, 1 tablespoon cornstarch
Boil for 10 minutes, stirring occasionally.
Use immediately or cool and store in the refrigerator for up to one week.
Notes
The spice level of this sauce comes from the amount of crushed red pepper flakes that you use. If you are adverse to spice I would suggest using 1 teaspoon, but if you like things spicy use the full 2 teaspoons.