Pre-heat your oven to 160°c (140°c for fan-assisted oven, Gas Mark 3 pr 325°F). Lightly grease a Lightly grease an 8 inch (20cm) loose-bottom tart tin. Set aside. In a large bowl, cream together the sugar and butter/margarine until light and fluffy.
Sift in the flour & corn flour, and mix to form a dough - you may need to use your hands. Tip the dough onto a floured worktop and gently knead until smooth.
Transfer the ball of dough to your prepared tart tin and push it to the edges, using your hands or the back of a spoon to evenly spread it across the base of the tin. If you want a decorative edge you can also press the edges with your finger. With a knife, gently score the top of your shortbread into triangles, without cutting right through. Prick the triangles with a fork/toothpick before transferring to your oven.
Bake for 45-55 minutes, until the shortbread is a light golden colour. Remove from the oven and cut through your score lines, whilst still warm and in the tin. Finish with a sprinkle of some caster sugar to finish.
Allow the petticoat tails to cool completely in the tin, before enjoying.