Pre-heat the oven to 170°C
Melt the butter in a small pan.
In a food processor combine the icing sugar, 2 tablespoons of caster sugar, flour and cornflour. Pour in the melted butter and process until combined.
Press into a 22cm springform tin. Use a butter knife to draw into 12 wedges. Prick each wedge lightly with a fork.
Bake for 35 minutes until golden.
Remove from the oven and sprinkle over 1 tablespoon of caster sugar. Let cool for 15 minutes in the tin before cutting into wedges while still warm.
Remove from the tin and transfer to a cooling rack.
Store in an airtight container.