Dissolve the cornstartch in water in a medium saucepan.
Add the remaining ingredients and stir well. Bring the mixture to a boil over medium heat, then reduce the heat and simmer for 5 minutes. Turn off the heat and cover the sauce until it has cooled.
Store the sauce in a covered container in your refrigerator.
3 cups water
2 teaspoons cornstarch
1 6-ounce can tomato paste
3 tablespoons white vinegar
4 teaspoons chili powder
2 teaspoons salt
1 teaspoon cayenne pepper