Put the flour, sugar and salt in food processor and pulse a few times.
Add the butter and process until the butter has completely dispersed.
MIx the egg yolk with the cold water.
With the motor running, add the egg-water mix in a steady stream. Stop the mixer when the pastry has formed a ball.
Remove the pastry from the processor and divide into two portions.
Shape the pastry into flat discs and wrap in cling film.
Chill pastry discs in refrigerator for at least 30 minutes.