Wash and pit the cherries (with a sturdy straw press the top of the cherry so that the pit comes out from the bottom).
Cut the cherries in half.
In a large saucepan, add the halved cherries, powdered sugar and lemon juice.
Heat over low-medium heat for about 35-45 minutes (depending on the water content of the fruit) stirring regularly, there should be a slight boil. Skim the surface from time to time with a slotted spoon. 5 minutes before the end of cooking, add the vanilla extract.
Tip: to know if the jam is set, pour a drop of the preparation on a plate taken out of the freezer. Then put the plate vertically: if the drop runs slightly then freezes, it's cooked enough! Otherwise continue cooking a bit.
Remove from the heat, if necessary, you can pass a little blender to have fewer pieces in the final jam.
Pour the mixture into sterilized jam jars. Close the jars and invert them for 10 minutes. Turn over until the jam has completely cooled.
Notes
This jam once opened can be kept in the refrigerator, about 10-15 days.