Sift all dry ingredients. Place in the bowl of processor.
Add chopped butter and whizz until it resembles breadcrumbs.
Add the egg yolk and whizz until mixture comes together to form a dough.
If needed add 1 tbsp iced water. This will depend on how dry your flour is, the weather (if it is humid and hot you probably won't need the extra water).
Turn out onto a lightly floured bench and form into a smooth ball. Don't knead the pastry too much or it will produce a tough pastry.
Wrap in cling wrap or place in a sealed container and rest in fridge for 1 hour.
Divide into a 1/3 and 2/3 balls, the larger for the base. Roll out the larger ball to fit your dish fill and then roll out the smaller ball, cut out a small circle from centre of pastry to allow steam to escape and place gently over you pie filling. Crimp pie edge to seal.
Brush the pastry with egg white and sprinkle with sugar just before baking. This gives a crunchy crust to the pie. Alternatively you can brush with a milk wash.
With a fruit filling bake at 350C for 45-50 minutes.
If you use the pastry for a slice base you can cover 2 slice tins.