Beat together the butter, sugars, and salt until light and creamy. Beat in the egg and vanilla, and fold in the flour., Divide the dough in half, shape both halves into disks, wrap in plastic, and refrigerate for at least 1 hour., Preheat the oven to 400°F. Grease (or line with parchment) two baking sheets., Working with one disk at a time, flour your work surface and roll the dough 1/4" thick., To shape cookies using a springerle pin: Brush a very light coating of flour onto the dough and your springerle pin. Slowly roll the springerle pin over the dough, pressing down hard enough to leave a good impression. Cut the cookies apart on the lines., To shape cookies using a springerle mold: Brush a very light coating of flour onto the dough and your springerle mold. Press the mold firmly into the dough, then remove and cut around the design with a knife or a fluted pastry wheel., Transfer the cut cookies to the prepared baking sheets. Bake the cookies for 10 to 12 minutes, until lightly golden around the edges. Remove them from the oven, and cool right on the pan., Store shortbread, well wrapped, for up to a week; freeze for longer storage. The flavor improves as the shortbread ages.