Preheat oven to 150°C. Grease a 20cm square cake tin.
Place nuts into a dry frying pan and place onto medium heat. Allow to toast until you can smell the nuts, careful not to burn them. Remove from heat and chop finely.
Beat butter and sugar until light.
Add flour, cornflour and nuts and bring together, it will look very crumbly.
Turn out into the cake tin and press dough into the tin.
Score the dough into narrow bars. Prick with a fork.
Bake for 40 Minutes until lightly brown around the edges of the tin.
Dust with extra castor sugar while still hot.
Allow to cool in the tin completely before removing from the tin and cutting into narrow bars.
Store in an airtight container.