• In the bowl of a food processor, combine the flour, confectioner's sugar, and salt. Add the cubed butter and process until crumbly. The texture will be like lightly-moistened sand.
• Add the egg yolk, water, and lemon zest (if using). Mix in the food processor until the dough comes together in a large ball. It is ok if there is a little loose dough, but it will basically be a whole ball.
• Turn the dough out onto a clean, lightly-floured surface. Knead it a few times into a smooth ball. Do not overwork the dough.

As a Tart Shell

• Using the palm of your hand or a rolling pin, flatten the dough into a flat, even disk about eight inches in diameter. Wrap and chill the dough for a least one hour, or up to two days.
• After the dough has rested and chilled, unwrap it and lightly dust both sides with flour. Roll the dough to about 11" diameter, so that there is enough dough to lay into the tart shell and cover the sides, plus a little overlap.
• Run the rolling pin or a paring knife over the rim of the tart shell to remove the excess dough, leaving a neat and clean rim. Dock the bottom of the crust all over with the tines of a fork. Loosely wrap and chill the shell for at least 30 minutes, or one hour.
• Preheat the oven to 350°F. Unwrap the tart shell. Line the tart dough shell with parchment or two layers of foil. (I prefer parchment.) Fill the shell with dried beans or pie weights, and bake for 30 minutes.
• Remove the beans or weights, and continue baking for 5 minutes more or so, until the tart shell is golden brown. Let cool fully before removing from the tart pan. Use a paring knife to gently pry away any stuck parts, but it should remove easily.

  PRO TIP: If you are making mini tarts, follow the same method, but bake without pie weights. Mini tart shells will take about 15 to 18 minutes to bake to a light golden brown.

As Cookies

• Follow steps 1 - 3, above. Shape the dough into a log about 1.5" in diameter and 10" to 11" in length. Wrap the dough in plastic wrap and chill fully, at least two hours or up to two days.
• Preheat the oven to 350°F and line two cookie sheets with parchment.

  Remove the plastic wrap from the chilled dough. If you'd like, lightly brush the outside of the log with egg and roll the log in sugar or colored sprinkles of your choice. If you want to use a variety of sprinkles (or make the log less unwieldy), just subdivide it into smaller logs.
• Slice the dough into 1/4" rounds. Place each slice on the cookie sheets, leaving about an inch of space between each.
• Bake for 15 to 20 minutes, until very light golden brown. Let cool for a few minutes on the cookie sheet before transferring the cookies to a cooling rack. The cookies will keep in an airtight container for a couple of weeks.