Cream together the butter and sugar, beating very well. Sift together the flour, cornflour and baking powder and beat into the butter and sugar.
Spread the mixture in a 30 cm (12 in) long baking tin, about 4 to 5 cm (1 to 2 in) deep and smooth even. Bake in a preheated oven, 180°C (350 F) for 20 minutes (or until golden brown).
Take the tin out of the oven and set aside to cool for 10 minutes while you make the caramel.
Put all the ingredients for the caramel - except the vanilla - into a saucepan. Over a moderate heat, stir until the butter has melted and the sugar dissolved completely.
Then bring the mixture to the boil and boil for 5 to 7 minutes. Take the pan off the heat, stir in the vanilla and continue stirring for 2-3 minutes to cool it slightly. Then pour it over the shortbread. Leave to cool completely.
- To finish, melt the chocolate over hot water (no overheat !!) and pour it over the caramel layer. When it is cool, mark into squares with a sharp knife (makes 12 - 16). Store in airtight container.