Mix the flour and salt in a large mixing bowl. Place it in the fridge for a few minutes until the flour feels cold. Starting out with cold flour makes better puff pastry.
Stir the butter into the flour using the blade of a dinner knife until each piece is well coated.
Add the water, then quickly use the knife to bring everything together.
Using one hand, grab the pastry in the bowl and place it on the work surface. Shape it to a very flat roll only by pressing on it, and not by kneading it. Wrap in cling film and refrigerate for 15 minutes.
Roll out the pastry on a lightly floured work surface in one direction to a rectangle about 1cm thick and measuring about 45x15cm. Keep the sides straight by shaping them with your hands. The top and bottom edges should be as square as possible.
Fold the bottom third of the pastry up and the top third down so it makes a 15x15cm piece. If your piece is slightly larger or smaller, that is OK too, as long as the corners are neat.
Rotate the pastry so that its open side is facing to the right side like a book. Use a rolling pin to press the edges together.
Repeat the rolling out and folding of the pastry as described, a total of four times. There will be little specks of butter scattered all over the pastry. If at any point the pastry feels too soft or starts springing back when you roll it out, it has become too warm. In that case wrap it in cling wrap and chill for 10 minutes, then continue the rolling and folding.
Wrap the pastry and chill for at least 1 hour, better overnight.
To use, roll the pastry out about 3mm thick. Brush with beaten egg yolk if desired (it gives it a nice golden colour) and bake at 200 C / Gas 6 until golden brown and puffed up, about 30 minutes.