Ingredients For the Puff Pastry 225g plain flour, plus extra for dusting
1/2 teaspoon salt
200g chilled butter, cut into 1cm cubes
For the Filling 500ml milk
1 vanilla pod, split down the middle and seeds scraped out
100g caster sugar
4 egg yolks
For the Icing 200g icing sugar
5 teaspoons water
50g dark chocolate, melted
In a large bowl, mix the flour and salt then rub in a third of the chilled butter with the tip of fingers until the mixture resembles fine breadcrumbs. Roughly rub in the remaining butter, leaving large lumps.
Pour in the water a little at a time until the pastry just binds together.
Transfer the pastry out onto a floured work surface and roll into a narrow rectangle about 2.5cm thick.
Place the pastry vertically in front of you, fold the bottom third up onto the middle third, then the top third down onto the other thirds.
Wrap the pasty with a cling wrap and place in the fridge to chill for 10 minutes.
Take out the chilled pastry and roll out again into a narrow rectangle and repeat the folding steps as before.
Chill again and repeat the rolling and folding steps one more time. Wrap the pastry and return to the fridge to rest.
Meanwhile, make the filling by pouring in the milk into a saucepan and add in the split vanilla pod and the seeds. Bring the milk mixture to the boil. Remove from the heat.
Whisk the sugar, egg yolks and cornflour in a large bowl. Pour in a little of the hot milk mixture, whisking continuously. Whisk in the rest of the hot milk mixture until well combined. Return to the pan.
Cook the mixture over a low heat, stirring continuously until thickens. Remove from the heat and pass the mixture through a sieve into a bowl. Add the butter and stir until melted and thoroughly combined. Leave to cook, cover with cling wrap and chill.
Preheat the oven to 220C. Line 2 baking trays with baking paper. Divide the pastry into 2 equal pieces and roll out both pieces to 20cm square and 5mm thick. Place each pastry sheet onto the baking trays and chill for 10-15 minutes.
Bake for 10-15 minutes until golden brown and crisp. Set aside to cool.
Line a deep 23cm square baking pan with foil and plenty extra at the sides. Place one pastry sheet in bottom of the tray. Spread the filling evenly before placing other piece of pastry on top. Refrigerate while making the icing.
Sift the icing sugar into a bowl and stir in cold water until combined. Set aside.
Transfer the melted chocolate into a piping bag fitted with a small plain nozzle. Set aside to firm up slightly.
Take the custard slice from the fridge and spread the icing over the top layer. Decorate with melted chocolate.
Place the slice back into the fridge to set. Cut into small pieces and slowly lift out of the tray onto a serving platter.