Bring the plums to the boil in a pan along with the lemon juice and vanilla seeds, reduce the heat, cover and cook over a low heat for approx. 15 mins. until soft, stirring occasionally. Puree the plums until smooth using a hand blender, leave to cool.
Mix the almonds, almond slivers, sugar and butter, fry in a non-stick frying pan over a medium heat until crispy, leave to cool.
Divide the plum puree between the jars, top with the crumble, seal the jars tightly.