Sift the flour and salt into a large bowl. Roughly break the butter in small chunks, add them to the bowl and rub them in loosely. You need to see bits of butter.
Make a well in the bowl and pour in about two-thirds of the cold water, mixing until you have a firm rough dough adding extra water if needed. Cover with cling film and leave to rest for 20 mins in the fridge.
Turn out onto a lightly floured board, knead gently and form into a smooth rectangle. Roll the dough in one direction only, until 3 times the width, about 20 x 50cm. Keep edges straight and even. Don’t overwork the butter streaks; you should have a marbled effect.
Fold the top third down to the centre, then the bottom third up and over that. Give the dough a quarter turn (to the left or right) and roll out again to three times the length. Fold as before, cover with cling film and chill for at least 20 mins before rolling to use.
Leftover trimmings should be stacked up and chilled or frozen for another use. Don’t scrunch them together in a ball or you will lose the layers. To re-roll, allow to come back to room temperature.