In a bowl, stir together the flour, cornstarch and sugar. Rub in the butter pieces, using your fingers, till the mixture resembles breadcrumbs. Gather this into a ball.
Place this ball in an ungreased 8u201d or 9u201d piedish, tart tin/ cake tin with a removable bottom. Press the dough out in the pan, firmly into an even layer. If this is not done, the shortbread will not firm up when baked and will crumble. With the tines of a fork, make decorative impressions around the edge and prick the surface evenly.
Bake at 160C for about 40 minutes or till the shortbread turns a pale golden brow. Do not allow it to brown any more or you will find that the bottom of the shortbread will be burnt!
Remove from the oven and sprinkle the sugar over the short bread. While still very hot, mark the shortbread with a knife into 8 or 12 wedges. Allow to cool completely then remove from the pan and serve with tea.