In a pan, on low heat, lightly toast the cashew nuts for a few minutes. Set aside to cool completely, then roughly chop into pieces. Preheat oven to 160°C (fan forced). Grease and line a 23x23cm springform pan with baking paper.
Make the shortbread layer: in a food processor pulse together butter, sugar and flour until a crumbly mixture is formed. Press mixture into baking pan and bake for 20 minutes or until just lightly golden in colour. Allow to cool while you prepare the caramel layer.
Bake for 10-12 minutes until lightly golden. Take out of the oven and allow the cookies to cool on the tray for 5-10 minutes before transferring to a wire rack to cool completely. Repeat with remaining dough.
In a saucepan, over med-high heat, add sugar and do not stir til the bottom layer of sugar begins to melt. Then stir continuously til it has turned light brown in colour and smooth in texture. Slowly pour in the thickened cream and stir for about 5 minutes. Note: at this point the caramel may clump into a ball, just keep stirring til its smooth and liquid again. Lower heat and stir in butter, salt and vanilla. Remove from heat and stir in cashew pieces.
Pour caramel layer into pan over the shortbread layer and bake for 15 minutes until bubbly. Remove from oven and cool completely before cutting into slices.