For the pastry, put the butter and flour into a food mixer and mix until the texture resembles breadcrumbs, then add the egg and mix until combined into a firm dough.
Wrap in cling film and place in the fridge to rest for 30 minutes.
For the tart, preheat the oven to 160C/325F/Gas 3.
On a work surface lightly dusted with flour, roll out the pastry to about 3mm thick.
Carefully line a 23cm/9in deep-sided, loose bottomed tart tin with the pastry, pressing the pastry into the edges of the tin. Spread the jam over the base of the tart, then place in the fridge to rest for 10 minutes.
To make the frangipane, beat the butter, vanilla seeds and sugar together in a bowl until pale and fluffy.
Crack in the eggs, one at a time, beating well after each addition, until all of the eggs have been fully incorporated into the mixture. Fold in the ground almonds carefully by hand.
Top the pastry case with the almond mixture and smooth to the edges, then decorate with the blanched almonds, in concentric circles.
Bake the tart in the preheated oven for 35-40 minutes, or until the filling has risen, is cooked through and the surface is pale golden-brown.
For the custard, put the milk and cream in a shallow saucepan with the vanilla pod leftover from making the frangipane filling. Set over a medium heat and bring to the boil.
Meanwhile, whisk the egg yolks and sugar together in a bowl.
When the milk is boiling, pour it onto the eggs whisking all the time, then return the whole mixture to the pan and cook over a gentle heat, whisking occasionally until thick enough to coat the back of a wooden spoon.
Strain into a clean pan and warm through.
To serve, cut the tart into wedges, place onto plates and pour the custard alongside.