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Separate an egg yolk from the white, and put it into a small bowl. Add the cold water and the vanilla extract. Mix briskly with a fork or mini whisk until the yolks break and turn smooth, then set aside. You can use all-purpose flour, but cake flour will give you even better results. Stir them briskly together using a fork or whisk until they are evenly combined. Cut the butter into -inch (0.64 centimeter) thick slices first, then combine it with the flour mixture. You can do this using a pastry blender, two crisscrossing knives, or in a food processor. The crumbs should be no larger than small peas. For best results, use cold butter straight from the fridge. Combine the two using a fork or rubber spatula. Do not overwork the dough, or the butter will become too soft. Once the dough has come together, transfer it to a lightly floured surface and gently pat it into a 6-inch (15.24 centimeters) wide disk. Wrap it up with some plastic wrap, and put it into the fridge for at least 30 to 60 minutes or overnight. The dough must be cold before you roll it out. Every so often, lift the dough and turn it by a quarter. Dust the dough with flour as needed to prevent sticking. Gently pat the dough against the bottom and the sides of the pan. Trim off any excess, overhanging dough until it is inch (1.91 centimeters) above the rim. This will prevent any shrinking. Try to use a fluted tart tin with a removable bottom. If you absolutely cannot find one, you can use a regular tart tin; you just won't be able to take the tart out of the tin at all. The pierced holes will prevent the dough from fluffing up during baking. Again, the dough must be properly chilled before you bake it; it would have softened up considerably during all that rolling. Alternatively, you can line the crust with tin foil or parchment paper just before baking, and fill it with pie weights, uncooked rice, or dried, uncooked beans. Preheat the oven to 350F (177C), and make sure that the baking rack is in the bottom third of the oven. Once the oven is hot enough, place the crust inside and bake it for 25 minutes, or until it turns golden brown. Once the crust is completely baked, take it out of the oven and place it onto a wire rack to cool off. Do not remove the crust from the tin.

Summary:
Combine the egg yolk, water, and vanilla extract, then set it aside. Combine the flour, sugar, and salt in a separate bowl. Cut the butter into the flour mixture until it resembles coarse crumbs. Blend the egg mixture into the flour mixture until the dough comes together. Form the dough into a thick disk, cover it with plastic wrap, and refrigerate it for 30 to 60 minutes. Roll the dough out into a 12-inch (30.48 centimeters) wide circle. Transfer the dough into a 9-inch (24.77 centimeters) tart tin with a removable bottom. Pierce the bottom of the crust a few times with a fork, then refrigerate it until it turns firm, again, about 30 minutes. Bake the crust in a preheated oven at 350F (177C) for 25 minutes. Set the crust aside to cool.