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1
Put the flour, sugar, and salt in a food processor and pulse to combine. Drop in the pieces of butter and pulse until the mixture looks like coarse meal-you'll have both big pea-sized pieces and small flakes. With the machine running, start adding the cold water gradually: add just enough water to produce a dough that almost forms a ball. When you reach into the bowl to feel the dough, it should be very malleable.
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2
Scrape the dough onto a work surface, form it into a square, and pat it down to flatten it a bit. Wrap the dough in plastic and chill it for about 1 hour (or for as long as 3 days). When you're ready to bake, center a rack in the oven and preheat the oven to 350u00b0F. Line a baking sheet with a silicone baking mat or parchment paper.
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3
Remove the dough from the fridge and, if it's very hard, bash it a few times with your rolling pin to soften it. Put the dough between sheets of plastic wrap or wax paper and roll it-or pat it-nto a rectangle that's about 1/4 inch thick and about 5 x 11 inches; accuracy and neatness don't count for a lot here. Transfer the dough to the lined baking sheet.
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4
Beat the egg yolk with a few drops of cold water, and, using a pastry brush, paint the top surface of the dough with the egg wash. Using the back of a table fork, decorate the cookie in a crosshatch pattern.
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5
Bake the cookie for 30 to 40 minutes, or until it is golden. It will be firm to the touch but have a little spring when pressed in the center-the perfect break-up is crisp on the outside and still tender within. Transfer the baking sheet to a rack and allow the cookie to cool to room temperature.
