In a large bowl, combine the flour and salt. Add the butter and toss to coat with flour. Using a pastry cutter or two knives, cut the butter into the flour until the butter is in 1/2-inch chunks. Sprinkle 1/2 cup very cold water over the flour-butter mixture and continue cutting it in until the mixture barely hangs together. Dump the dough onto a lightly floured surface and gently press into a rectangle. WIth a floured rolling pin, roll into a rectangle, approximately 6 inches by 18 inches. Don't worry if the dough is very rough & ragged at first - this is normal.
With a pastry scraper or large spatula, fold both ends in to meet in the middle, then fold in half like a book. Roll out again into a 6 x 18-inch rectangle, repeating the folding technique as before. Wrap the dough tightly in plastic and chill for at least 20 minutes.
Remove the chilled dough from the refrigerator and return to the lightly floured work surface. Roll out to 6 x 18 inches once more, folding the dough as before. At this point, the surface should be smooth with large bits of butter visible throughout. Cut the puff pastry in half. Wrap tightly in plastic and refrigerate until well chilled, about 2 hours or overnight. This pastry freezes exceptionally well and may be kept frozen for several months.