Sieve the flour and form into a mound on the work surface. Make a well in the centre. Dissolve the salt in the water and pour into the well. Gradually mix together the flour and water using your fingers. When the ingredients are thoroughly mixed, work the dough with the palms of your hands until completely smooth. Wrap the dough in a cloth or greaseproof paper and leave to rest in the refrigerator for at least 2 hours.
Dust the work surface with a little flour and roll out the dough to form four flaps of about 10 cm/4 inches wide, each at right angles to one another, and leaving a thick piece of dough in the middle. Knead the butter to form one supple slab; it must still be cold as well as supple. Place this in the centre of the dough. Fold over each of the flour flaps to completely enclose the butter. Return to the refrigerator, covered, for a further 30 minutes.
Again, lightly flour the work surface and gently roll the dough into a rectangle about 60 cm/24 inches long and 40 cm/16 inches wide. Keep the surface of the table dusted with flour while rolling and always roll the pastry away from yourself. Mark the rectangle into three sections. First fold one end section over the middle section and then fold the final section over the other. This is called a ‘turn’. Now turn the dough through 90 degrees and roll it out again into a rectangle as before, flouring the work surface as you go. Once again, mark out three equal sections and fold over as before. The pastry has now had two ‘turns’. Cover the pastry and leave to rest and firm up again in the refrigerator for at least 30 minutes.
Give the pastry another two ‘turns’ exactly as the first two, cover and return to the refrigerator for another 30 minutes. Give the pastry two final ‘turns’ so that it has now had six ‘turns’ in total. After resting again for 30 minutes, the pastry will be ready for use. It will last for about 4 days in the refrigerator or, if you wish, freeze it and then you can keep it for a few weeks.