1 3/4 cups flour
2/3 cup sugar
1 tsp coarse salt, plus more for sprinkling if desired
9 Tbsp cold unsalted butter, cut into 18 pieces
3-5 Tbsp cold water
1 egg yolk, plus additional water
Put the flour, sugar and salt in a food processor, and pulse for a few seconds to combine. Add the butter, and pulse until you're left with pea-sized bits (in addition to some small flakes). Pulse the machine and add the water gradually, until it just barely forms a ball (this might not require the whole amount). The dough will be very soft.
Scrape the dough onto a piece of plastic wrap, form it into a chubby square, and refrigerate for at least an hour (or a few days, if needed).
When the dough has chilled, preheat the oven to 350 degrees. Line a cookie sheet with a silpat or parchment, or grease well. Let the dough soften at room temperature for a few minutes. Roll out (between plastic or waxed paper) into a rectangle that's about 5"x11", and transfer to the prepared sheet.
Beat the yolk with a splash of water, and paint it over the surface of the dough with a pastry brush (or, if you're me, a wadded up bit of the waxed paper you used to roll out the dough). Using the back of a fork, decorate the cookie in a crosshatch pattern by drawing the underside across in one direction, forming a series of tracks, then perpendicular to them. Sprinkle with additional salt, if you favor a pronounced salty-sweet flavor.
Bake the cookie for 30 to 40 minutes, or until it is lightly golden (and slightly dark on the edges, if that's what you favor). It will be firm to the touch, but have a little spring when pressed in the center. Allow to cool to room temperature, then break it as you please, either before serving or at the table.