Put the flour, salt, and chilled butter into a large bowl and rub them together with your fingertips until they resemble fine bread crumbs. Make a well in the center of the mixture and pour in the water. Mix with a knife, then bring the dough together with your hands.
Squidge into a ball, then wrap in plastic wrap and put in the refrigerator for 25 minutes.
Unwrap the dough ball and use a knife to score a large cross in the middle of it, cutting no more than halfway through. Lift all four corners from the middle of the cross, then pull them up and out to make the cross big enough to put the softened butter into.
Add the butter, then fold the corners of the cross back to the center, covering the butter so it is completely enclosed. The corners should overlap in the center so no butter is showing. It is important that the butter is not too hard or too soft, otherwise it will escape through the dough when you roll it out and the resulting pastry will not rise as well. If it's too hard, let the ball of pastry stand at room temperature until the butter inside has softened; if it is getting too soft, pop it in the refrigerator for 20 minutes to firm up a little before rolling.
This next process is called "rolling and folding" (or "turns") and it creates the characteristic flaky layers of puff pastry. Begin by rolling the pastry out away from you on a well-floured counter to a rectangle roughly 3 times as long as it is wide (don't turn the pastry when rolling). Keep the corners square and edges straight by pressing a spatula or rule against them. Lift the dough occasionally to make sure it isn't sticking; flour the counter again if necessary and sprinkle with more flour as you go, dusting away any excess with a pastry brush. Take the short edge of the pastry nearest to you and fold it up a third, then fold the top edge down a third to give a rectangular block. Turn the dough 90 degree and then repeat the rolling and folding.
You have now given the dough two "rolls and folds." Wrap the dough in plastic wrap and put in the refrigerator. Chill for 20 minutes.
Remove the dough from the refrigerator, then unwrap and give it two more "rolls and folds." Wrap and rest in the refrigerator for at least another 20 minutes. The block of puff pastry can at this point be kept in the refrigerator for a day or two, or frozen.
Remove from the refrigerator, then unwrap and give the dough a final couple of rolls and folds. Roll it out to the size desired for your chosen recipe. Place on a baking sheet, then cover with oiled plastic wrap and let rest in the refrigerator for about 30 minutes before using.