For the rich shortcrust pastry, sift the flour with the salt into a large bowl. Rub in the butter until the mixture resembles breadcrumbs. Stir in the icing sugar.
Mix the egg yolk with 2 tbsp of the water and add to the flour mixture.
Mix to a firm dough, first with a knife, and finally with one hand. It may be necessary to add more water, but the pastry should not be too damp.
Roll out on a lightly floured surface and use to line a 23cm pie dish. Prick the base with a fork, then trim and crimp the edges. Chill for 20 mins.
Heat the oven to 170°C/150°C fan/325°F/Gas 3. Line the pie crust with baking parchment and fill with baking beans. Bake for 20 minutes, then remove the baking beans and paper. Return to the oven and bake for a further 10 minutes until the base is dry. Remove from the oven and leave to cool. (Cool for 5 minutes then freeze for 10 minutes)
For the crème patisserie, scald 400ml of the milk by bringing it just below boiling point in a saucepan.
Cream the egg yolks with the sugar and the remaining 50ml of milk and, when pale, mix in the flours. Pour on the hot milk and mix well.
Return the mixture to the pan and bring slowly to the boil, stirring continuously. (It will go alarmingly lumpy, but don’t worry, keep stirring vigorously and it will become smooth.) Simmer for 2 minutes. Leave to cool slightly, then add the vanilla essence.
Cover with greaseproof paper to prevent a skin forming and chill.
Whip 150ml of the double cream to soft peaks, then whisk into the crème patissiere.
Pour the crème patissiere into the cooled pie crust and smooth with a palette knife. Chill until set. Remove the pie from the pie tin and place on a cake stand.
Whip the remaining 300ml double cream to soft peaks and spoon into a piping bag fitted with a star nozzle. Pipe cream around the edge of the custard pie.