1. Lightly butter 2 trays, or line with baking parchment.
2. Put the butter and sugar into a large bowl and cream together, using an electric hand-held whisk or wooden spoon, until light and fluffy. Sift the flour and cornflour into the bowl, add the salt and mix together until smoothly combined. Tip the mixture out onto a lightly floured surface and knead to a soft dough.
3. Roll out the dough between 2 pieces of baking parchment to a thickness of 1cm. Prick the dough all over with a fork and cut into triangles or whatever shapes you like, using a knife or a biscuit cutter. Re-roll the scraps once to cut more (if you re-roll too many times the dough may start to get greasy). Put the shortbreads on the prepared baking trays and chill for at least 30 minutes.
4. Meanwhile, heat your oven to 170°.
5. Bake the shortbreads for about 20 minutes, until just turning golden brown at the edges. Leave on the baking trays for a few minutes to firm up slightly, then lift the shortbreads onto a wire rack. Dust with sugar and leave to cool. They will keep in an airtight container for 3-4 days.