Pour the flour, salt and butter into the bowl of the food processor fitted with the flat beater. The butter must be cold (I put it in the freezer for about 10 minutes). Mix for about 2 minutes (you will still have pieces of butter, it's normal).
Add cold water. Mix for another 2 minutes.
Shape the dough into a ball and place it on a floured counter.
Roll out the dough into a rectangle. Make 4 simple turns (flour your work surface and the dough well, it will stick). For details of that step check the inverse puff pastry recipe.
Wrap the dough in plastic wrap and leave it in the fridge for at least 1 hour.