This is a classic French pie pastry, which works well for both sweet and savoury dishes. It's almost the same as traditional short crust pastry - which uses one portion of fat to every two of flour - but uses egg and a higher fat ratio (64% rather than 50%) and rolls much better.
As with all pastries, it's vital not to overwork the dough. Just bring it together with a spoon, and when using your hands, use your fingers rather than the heel of your palm as you would with bread dough, as the heel is the warmest part of your hand. It's also essential to chill the dough well before you roll it out.
5g salt (or one level teaspoon)
250g plain flour
1 tbsp cold water
Use chilled butter straight from the fridge and cut it into small squares. Sieve the flour and salt into a bowl and add the butter. Using the tips of your fingers, crumble the flour and fat together to make breadcrumbs.
Then add the egg and water, and using a spoon, work it all together into a ball. Wrap in clingfilm and leave in the freezer for 20 minutes.
Roll out the pastry to about the width of a £1 coin and use for tarts, pasties, pies etc.