Place the sugar, butter and vanilla seeds in a planetary mixer with paddle attachment.
Work until you have a smooth, light-coloured mixture, then add the eggs, the flour and the lemon zest.
When you have a uniform mixture, cover with clingfilm and leave to rest in the fridge for at least 1 hour.
Peel and dice the apple and transfer to a bowl of water with a squeeze of lemon juice to stop it turning brown.
Drain the apples and transfer to a pan, also adding the sugar, cinnamon, rum, pine nuts and empty vanilla pod.
Cover with a lid and cook over a low heat until the apples are tender.
When they are cooked, roughly mash and transfer to a pastry bag.
Once it has rested, roll out the shortcrust pastry dough to a height of 4 mm with a rolling pin and then cut out disks using a pastry cutter or glass.
Cover half the disks with the apple filling and close with a second shortcrust pastry disk, carefully sealing the edges.
Arrange the biscuits on an oven tray lined with baking paper and cook for about 20 minutes at 180 °C until golden.
Dust with icing sugar and enjoy together with a nice cup of tea.