To make the pastry. Using a mixer, beat the sugar and butter in a bowl until smooth and creamy but not fluffy. Scrap out vanilla seeds and add to mix. With mixer on a slow speed, gradually incorporate the beaten eggs.
Sift the flour and salt together and add to butter mix. Add flour in three or four stages. As the mix comes together in a crumbly dough, stop the machine.
Gather the dough together and turn out onto a lightly floured surface. Briefly knead until smooth and wrap in cling film. Leave to rest in refrigerator for 30 minutes before use.
Grate the pastry into a tartlet base. Forcing the mix down the heel of your hand. Cover the base and force the pastry up the sides of the base with exess pastry. Place in fridge for 20-30 mins. Preheat oven to 180°C and bake blind for 15 mins.
To make the tart. Boil prunes until cooked. De-stone. Place milk, cream and lemon zest in a saucepan. Slit vanilla seeds from pod and add seeds and pod to mix.
Heat slowly to the point of boiling, then take off the heat, stir in the sugar and leave aside for 30 mins. Cut 2/3 of the prunes into small chunks and scatter over the pastry base. . Beat the egg yolks and full eggs into a bowl.
Remove lemon zest and vanilla pods from creamy milk. Bring mix back to the boil. Pour slowly onto the beaten eggs while whisking.
Put the remaining prunes into a food processor and whizz lightly to a pulp. Slowly pour in the custard until evenly blended. Pass through a sieve.
Return pastry case to the oven and pull the oven shelf out as far as is safe. Slowly pour in the prune custard mix up to the top. Bake tart for 50min-1hour at 150°C until the custard barely wobbles when shaken.