For the shortbread, preheat oven to 180 C / Fan 160 C / Gas 4. Grease a 23x27cm (9x11 in) baking tray. Cream butter and sugar until light and fluffy. Sieve over the flour and mix until the mixture binds. Press mixture into baking tray and smooth the top. Bake for 20 minutes.
Place the ingredients for the filling in a pan and heat gently, stirring until the sugar has dissolved. Bring to the boil, still stirring, and simmer for about 20 minutes until the mixture is thick. Leave to cool slightly, then spread over the shortbread. Leave to set
For the topping, melt the dark chocolate and the butter in a heatproof bowl set over a pan of gently simmering water, and stir until melted. Pour over the caramel and leave to cool. Melt the white chocolate in the same way, them pour into a small cone made of greaseproof paper. Snip off the end, then decorate the top of the cake with drizzles of white chocolate. Leave to set, then cut into squares.
Keep an eye on the caramel whilst its cooking as the sugar tends to stick the the bottom of the pan even with non-stick pans, it's a little harder with electric cookers as they take longer to cool for the simmering, so have another ring on low ready for this.
Just like the one my mum used to make for me!!
Tip: To know when the caramel is done put a tiny bit on a cold plate and leave to cool for a minute and if the caramel is ready it should have set - 15 Jan 2012