For the base: In a plastic mixing bowl zap butter in microwave for ten or twenty sec's till soft. Add caster sugar & vanilla. Mix well, sift in flour and mix. Get your hands in and mush it up till it's lovely and soft.
Grease up your baking tray & spread in the base mixture till it fits the whole tray. Prick holes before you put in oven on 170 C / gas mark 3 and cook for 30-40 mins till golden brown.
For the filling: Put butter, milk and syrup in a heavy duty saucepan bring to the boil (you must keep stirring). Turn down heat & simmer for 1-2 hours (while still stirring) till it reaches soft-ball stage. When cool pour on shortbread & put in fridge to set.
Melt a 300g bar of chocolate and pour on top of the caramel. Sprinkle chopped nuts, put back in fridge to set for 10 mins, cut up and enjoy.