Preheat oven to 160°C - line baking tray with baking paper and set aside.
Beat together butter and caster sugar until mixture is pale and creamy.
Mix through plain flour and cornflour until dough starts to come together.
Add lemon zest and rosemary leaves to the dough and combine everything well.
Transfer dough to tray and then press out to a round about 1/4" thick.
Give it a fancy edge by pressing gently with fork prongs, or use your thumbs to create a fluted edge.
Score the shortbread into 8 segments with a sharp knife - but don't cut all the way through.
Prick the shortbread with a couple of times in each segment so that the middle cooks evenly all the way through.
Place in oven and bake for 30 minutes or until turning lightly golden. Remove from the oven and then sprinkle the top with a little additional sugar. Set aside on the tray to cool completely.