1. Wash, dry, and pit the cherries. Try to save all the juices.

2. In a small bowl mix sugar and cornstarch. In a large pot mix water, lemon juice, zest and vanilla extract (optional), then add the cornstarch-sugar mixture and mix until there are no more lumps. Bring the mixture to a boil, then add the pitted cherries and let them simmer for about 8-10 minutes. The cherries should be soft but not mushy and the juice should have thickened.
If you want to use the filling right away, let it cool down a bit, then use it in your pie recipe as directed. You can store the filling for up to 2 days in the fridge in an airtight container.

3. If you would like to store the cherry filling for longer, you have to »cook« it once more. Fill the cooked filling into heatproof glass jars, leave about 2,5cm space to the top of the glass jar, close them and place them in a large pot with water. The water should be as high as the filling in the glasses. Bring the water to a boil and cook the glass jars for about 30 minutes. Remove the glass jars from the pot, check the lids and make sure they are closed. Be careful, they are very hot! Let the glass jars cool down completely on a kitchen towel. Store in a dark, cool place for up to 6 months.