In a plastic mixing bowl microwave the butter for ten or twenty seconds until soft. Add caster sugar and vanilla essence. Stir well then sift in flour and mix. Knead with your hands until it is nice and soft.
Grease a 23 x 33cm baking tray then spread in the base mixture until it fits the whole tray. Prick holes over the surface with a fork, then bake for 30-40 minutes until golden brown.
To make the caramel filling:
Put butter, condensed milk, sugar and golden syrup in a saucepan and bring to the boil, stirring constantly. Turn down heat to low and simmer for 1-2 hours (stirring occasionally) until it reaches soft ball stage. When cool, pour the filling over shortbread and put in fridge to set.
Melt the chocolate in a double boiler or the microwave and pour on top of the caramel filling. Sprinkle chopped nuts if you like then put back in fridge to set for 10 mins. Slice and enjoy!