First prepare the dough. If making the pastry by hand, sift the flour into a large bowl and add the salt, butter and water. Using your fingers, gently mix to an even dough. Transfer the dough to a lightly floured surface and knead it for five minutes or until smooth. If making the pastry using a mixer, sift the flour into the mixer bowl and add the salt, butter and water. Using a dough hook, work at a medium speed for a few minutes to incorporate the butter into the flour until it forms a smooth dough.
Flatten the dough into a neat rectangle, wrap it tightly in cling-film and rest in the refrigerator for 45 minutes.
Meanwhile, prepare the lock-in butter. Thoroughly clean and dry the bowl and line a baking tray with parchment paper. Place the butter and flour in the bowl and either work the flour into the butter using a wooden spoon for five to 10 minutes, or use the mixer working at a low speed for two minutes or until everything is well incorporated. Scrape the butter mixture onto the lined tray and, using floured hands, shape it into a square about 1cm thick. Place the butter mixture in the refrigerator until just chilled but not completely hard. (It is important that the chilled dough and lock-in butter are similarly firm, otherwise they will not roll together evenly and this may cause rips and holes in the dough.)
On a lightly floured surface, roll out the dough into a large square that is twice the size of the lock-in butter. Place the butter in the centre of the dough but at an angle so that the corners point towards the edges of the dough. Making sure you do not trap any air, fold the corners of the dough over the butter, bringing them into the middle like an envelope. Lightly pinch together all the joins to seal and completely encase the butter.
Roll out the dough and butter into a rectangle roughly three times as long as it is wide, using the sides of your hands to make sure the edges are neat and square. Dust any excess flour from the surface of the dough. With the shortest side closest to you, visually divide the dough horizontally into thirds and very lightly dampen the centre third with a little water, then fold the bottom one third of the dough over the centre third. Repeat by folding the remaining top third over the double layer of dough, then tightly wrap the dough in cling-film. Lightly press your finger into the bottom right-hand corner of the dough to make an indentation which signifies the first turn and how the dough was positioned on the board before you put it into the refrigerator.
Chill the dough in the refrigerator for 30 minutes. (Chilling the pastry between each roll out and fold allow the butter to harden so that clean, even layers of dough and butter are built up.)
Unwrap the dough and place it on a lightly floured surface with the indent in the same position as before at the bottom right-hand corner. Next, turn the dough 90 degrees clockwise. Roll out the dough into an 18cm by 25cm rectangle and repeat the folding process. Make sure all corners and sides are straight. Wrap the dough in cling-film again and this time make two indents on the dough in the bottom right corner. Chill in the refrigerator for a further 30 minutes.
Repeat the turning and folding processes two more times, each time chilling the dough for 30 minutes and marking with indents in the bottom right-hand corner to make sure the dough is turned in the correct direction. After the final turn, chill the dough in the refrigerator for 45 minutes and then it is ready to use.