Whisk together the flour, powdered sugar and salt. Cut the butter into the flour mixture using a pastry cutter, fork or your fingers, until the biggest pieces are the size of peas. Place mixture in the freezer for 5 minutes. Make a well in the center, then add the egg and stir with a for until it just starts to come together. Continue stirring until the dough starts to form clumps, then pour it out on to a lightly floured surface and knead ever so gently until all the dry ingredients are incorporated. Form into a disc, being careful not to overwork, wrap and refrigerate for at least an hour (up to overnight).
Roll out the chilled dough on a lightly floured surface until it reaches 10-11 inches in diameter. Transfer the dough to a 9-inch tart pan with a removable bottom. Seal any cracks that occurred in the transfer, then fold the overhang in to form double-thick sides, then run the rolling pin over the top to trim off any excess and make them an even height all around. Prick the bottom with a fork, then transfer to the freezer for at least 30 minutes prior to baking.
Preheat the oven to 375 degrees and butter the dull (not shiny) side of a piece of tin foil and press it firmly against the crust. Bake for about 15 minutes, remove the foil, and bake for another 5-10 minutes until its just starting to turn golden blonde. Remove from the oven and allow to rest for at least 30 minutes.