In a small bowl, whisk the beaten egg yolk, water and vanilla; set aside.
Put the flour, sugar and salt in the bowl of a food processor and pulse until combined. Add the butter cubes and pulse until the mixture resembles oatmeal with some larger (roughly pea-size) clumps. Drizzle the yolk mixture over the flour mixture and pulse until dough just forms a ball.
Transfer the dough to a work surface, pat into a ball and flatten into a disk. Use the dough immediately, or wrap in plastic wrap and refrigerate until well chilled, about 30 minutes. (The tart dough may be made ahead and frozen for up to 1 month.)
To roll out the dough, on a lightly floured board, flatten the disk. Gently lift the dough and return it to the surface. Lightly dust the top of the dough or the rolling pin with flour as needed, then roll out until the dough is about 1/8" thick, gently lifting and giving the dough a quarter turn occasionally. Use a small, sharp knife to cut out a round or rounds 2 inches greater in diameter than your tart pan.
Makes enough dough for one 9 1/2" tart, six 4" tartlets, twelve 2" miniature tartlets or one 13 3/4" x -4 1/4" rectangular tart.