On a lightly floured surface, roll puff pastry to 1/8-inch thick and trim to a 2 by 12-inch-rectangle. Transfer to a baking pan. Prick well with a fork and chill for 20 minutes. In a small bowl, beat egg white and confectioners' sugar together with a wooden spoon until smooth. Using a small offset spatula, spread a thin layer onto the chilled pastry. Freeze for 25 minutes. Heat oven to 425 degrees F. Remove iced pastry from freezer. Using a damp knife, cut vertically into 16 (3/4-inch) fingers. Transfer to a baking pan. Bake until pastry is golden and icing is curled up at ends, about 15 minutes. Transfer to a wire rack to cool.
By hand, work together 1 pound plus 4 tablespoons of the butter with 1 1/4 cups of the flour on a cool surface. When well mixed, form this butter paste into a block, wrap in plastic wrap and refrigerate. Rub the remaining butter into the remaining flour with the salt until the mixture resembles fine bread crumbs. Add 1 cup plus 3 tablespoons water and lemon juice to form a dough, and mix until smooth and elastic. Shape the dough into a ball, then cut a cross in the top to 1/3 of the depth. Open out the points of the cross and roll out each 1 to a square 5 by 5-inches, 1/4-inch-thick. Remove the butter block from refrigerator, and place in the center of the dough. Fold the flaps over, folding counter clockwise and ensuring that the edges are well sealed. Roll out the dough into a 12 by 24-inch rectangle. Fold the short sides to meet in the middle, then fold in half like a book to form 4 equal layers; this is called a double turn. Cover with a damp cloth and leave to rest in the refrigerator for at least 30 minutes. Repeat to give the dough 4 more double turns, resting in the refrigerator each time, then leave the dough covered in the refrigerator to rest, preferably overnight before using. Freeze for future use. Yield: about 3 1/2 pounds
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