Mix flour and salt in a bowl and rub in one-third of the butter until crumbly.
Rub in remaining butter leaving large lumps visible.
Add water gradually until the pastry binds and roll into a 2.5cm thick rectangle.
Fold the pastry into thirds and chill in the fridge for 10 minutes.
Repeat the rolling, folding, and chilling process two more times for a total of three turns.
Boil milk with vanilla pod and seeds, then remove from heat.
Whisk sugar, egg yolks, and cornflour together in a large bowl.
Whisk hot milk into the egg mixture gradually and return to the pan.
Cook over gentle heat until thickened and just boiling.
Sieve the custard, stir in butter until melted, and chill covered with film.
Pre-heat oven to 220°C and line two baking trays.
Roll two 20cm pastry squares to 5mm thickness and chill for 15 minutes.
Bake pastry for 15 minutes until golden and allow to cool.
Line a 23cm square tray with foil and place one pastry sheet inside.
Spread custody evenly over the pastry and top with the second sheet.
Mix icing sugar with water and spread over the top pastry layer.
Pipe parallel chocolate lines and use a toothpick to create a feathered pattern.
Chill until set, then cut into 8 pieces and serve.