In a large bowl mix the flour and salt together. Rub in a third of the butter until the mixture resembles fine breadcrumbs. Then roughly rub in the remaining butter, leaving large lumps.
Add the water a little at a time until the pastry just binds together (you may not need all the water).
Tip the pastry out onto a floured work surface. Roll into a narrow rectangle about 2.5cm/1in thick.
With the pastry vertically in front of you fold the bottom third of the pastry up onto the middle third then the top third down onto the other thirds. This is called a turn.
Wrap the pastry in cling film and place in the refrigerator to chill for 10 minutes
Take out of fridge and with a rolling pin roll out again into a narrow rectangle and repeat the turn as before.
Chill again and repeat the rolling and turning once more, so a total of three times. Wrap the pastry in cling film and return to the fridge to rest.
While the pastry is resting, make the crème pâtissière. Pour the milk into a pan and add the split vanilla pod and its seeds. Bring the milk mixture to the boil, then remove from the heat.
Whisk the sugar, egg yolks and cornflour together in a large bowl.
Pour out a little of the hot milk onto the egg mixture, whisking continuously. Whisk in the rest of the hot milk until well-combined, then return to the pan.
Cook the mixture over a gentle heat, stirring continuously, until the mixture becomes thick. It will just come to the boil.
Remove from the heat and pass the mixture through a sieve into a clean bowl. Add the butter and stir until melted and thoroughly combined.
Leave to cool, cover with clingfilm and then chill before using.
Pre-heat the oven to 220C/425F/Gas 7. Line two baking trays with baking parchment.
Divide the pastry into two equal pieces and roll out both pieces to 20cm/8in square and 5mm/¼in thick. Then place each pastry sheet onto the lined baking trays, and chill for 10-15 minutes.
Bake the pastry sheets for 10-15 minutes or until golden-brown and crisp. Set aside to cool.
While the pastry bakes, line a deep 23cm/9in square baking tray with foil with plenty of extra foil at the sides. The extra foil allows you to lift out the assembled slices.
Place one pastry sheet in bottom of the lined baking tray. (Reserve the prettiest piece for the top.)
Spread the crème pâtissière evenly onto the pastry in the baking tray before placing other piece of pastry, on top. Refrigerate while making the icing.
For the icing, sift the icing sugar into a bowl. Stir in cold water until thoroughly combined and set aside.
Transfer the melted chocolate into a piping bag fitted with a small plain nozzle, and set aside to firm up slightly.
Take the custard slice from the fridge and spread the icing over the top layer of pastry.
Using the piping bag, draw ten parallel lines along the top of the icing in one direction. Using a tooth pick, pull parallel lines about 2.5cm/1in across the melted chocolate and icing in alternating directions to create a feathered effect.
Place the slice back into the fridge to set.
Cut the finished vanilla slice into eight pieces.
Using the foil carefully lift the portioned vanilla slices out of the tray and place onto a serving platter.