In a medium bowl combine the flour, powdered sugar, and salt.
Cut the cold butter into cubes and toss in the flour. Using your fingers, cut the butter into the dry ingredients until pea sized pieces are left.
In a small bowl, whisk together the egg yolk and milk. Add to the dough, and use a rubber spatula to mix until cohesive dough forms.
Shape the dough into a disk, cover with plastic wrap, and chill for at least one hour or overnight.
On lightly floured parchment paper, roll the chilled dough out to ⅛” in thickness. Flip the dough over onto a 9” tart pan and coax it to fit up against the sides. Cut the excess dough off, then cover with plastic wrap, and chill for an additional hour.
Near the end of the chill time, preheat the oven to 375 degrees F.
Dock the chilled dough with the tines of a fork all over the bottom and sides. Line the dough with aluminum foil, then fill with pie weights, concentrating the weights on the edges to hold up the sides.
Place the tart pan on a baking sheet, then bake the shortcrust pastry for about 16-18 minutes or until the edges are lightly golden and the center no longer looks doughy. Set aside to cool completely.
Turn down the oven to 325 degrees F.