In a large bowl, whisk together flour, confectioners’ sugar, and salt. Using a pastry cutter or your hands, cut in cold butter until mixture resembles coarse meal. Add egg, and stir until combined. Press firmly with your hands until dough comes together and is fully hydrated, being careful not to knead too much. Shape dough into a flat disk, and wrap in plastic wrap. Refrigerate for at least 1 hour, preferably overnight.