Sandwich the softened butter between two sheets of parchment or waxed paper. Flatten with a rolling pin as evenly as possible to form a roughly 4x5-inch rectangle. Transfer to a baking sheet (without removing the paper) and refrigerate while you make the dough.
In a medium bowl, combine the flour and salt. Pour in the melted butter and lemon juice. Add a little less than 1/2 cup water, and mix all the ingredients with your hands. Add more water, if necessary, to make a soft dough. Knead until you have a smooth dough, about 1 minute.
On a lightly floured surface, roll the dough into an 8x10-inch rectangle. Remove the top layer of paper from the butter and invert the butter in the center of the dough so that the corners of the butter rectangle point toward the centers of the sides of the dough rectangle. Remove the other layer of paper. Fold the corners of the dough over the butter, like an envelope, covering it completely. Transfer to the baking sheet, cover, and refrigerate for 30 minutes.
Dust the counter lightly with flour and lay down the butter and dough envelope, seam side up. Lightly dust the top with flour and roll out to form a 12x15-inch rectangle. Don’t turn the dough as you do this. If any butter breaks through the pastry, just dab it with a little flour and carry on.
Lift one short end up and over the central portion of pastry, then fold the other end over the central part, as if you were folding a letter. Press the edges down lightly with a rolling pin to seal. Turn the pastry clockwise 90 degrees. What was the top edge of your pastry will now be on the right. Mark the top left corner by pressing your finger into it. You have completed the first “turn.” The pastry and butter should still be firm enough to roll another turn, but if it is very soft and the butter is on the verge of oozing, put it on the baking sheet, cover, and chill for 20 minutes.
For your second turn, dust the work surface again with flour and put the pastry on it, with the marked corner at the top left. Dust with a little flour and roll the pastry out again, without turning the dough, into a 12x15-inch rectangle. Fold the pastry as before and turn it clockwise 90 degrees. Mark the top left corner with 2 indentations of the finger (this is to remind you that you have completed two turns). Transfer the pastry to the baking sheet, cover, and chill for 30 minutes.
Make another 4 turns, covering and chilling the dough for 20 minutes after each turn or every other turn, as needed. At the beginning of each turn place the pastry in front of you with the marked corner at the top left. After the third turn, make three indentations in the top left corner, and so on.
Wrap the pastry in plastic and chill for an hour (or longer) before using. Let the pastry sit at room temperature until pliable before rolling, about 10 minutes.
Complete the last turn, then wrap tightly in plastic and refrigerate for up to 2 days or freeze for up to 2 months. If freezing, thaw completely in the refrigerator before using.