This prune filling is a cooked, thick jam or spread (like apple butter) that can be used to fill cakes, cookies or used as a jam on toast or muffins. Growing up I ate delectable prune-filled pastries thanks to my Slovak grandma! Smelling this while it is slowly cooking on the stove reminds me of her. Keep this covered in the refrigerator for up to 3 weeks. It can also be frozen if you aren't going to use it all at once.