Preheat oven to 150 degrees.
Place the butter, icing sugar and lemon zest into a cake mixer with paddle attachment. Beat for 5 minutes until creamy and fluffy. Sift the flour and cornflour together. Add to the creamed butter and mix to form a smooth dough.
Lightly flour your bench. Roll out the dough to a rectangle, measuring approximately 25cm x 35cm and 5mm thick. Cut the shortbread into rectangles, and place on a lined baking tray. Sprinkle sugar over shortbread and bake for 25 minutes until lightly golden around the edges.
Remove from oven and cool. Store in an airtight container.