Grinding: Add flour, salt, sugar (if you are making a sweet galette), cold butter cubes and egg yolk to the bowl of a food processor. Mix everything with a few pulsating strokes until you get a smooth crumb.
Adding water: Add ice water one tablespoon at a time, stirring briefly each time in pulsating mode. The dough should start to stick together.
Aging: Shape the dough into a ball, wrap in cling film and place in the fridge for 30 minutes to harden the butter and mature the dough.