Preheat the oven to 180°C (160°C Fan/325°F/Gas Mark 3). In a bowl, beat the butter and sugar together until soft and creamy. Add the flour and salt. Carefully mix together and then beat until well combined.
Press into an 18 x 28cm (7 x 11 in) oblong cake tin, 4cm (1 ½ inch) deep. Prick all over with a fork and bake in the oven for about 30-40 minutes until pale golden and firm. Allow to cool in the tin.
Put the butter, syrup and condensed milk in a saucepan and heat gently until mixed. Raise the heat and bring the mixture to a gentle boil, stirring all the time, and simmer gently for about 5-7 minutes until the mixture has thickened and is a creamy fudge colour. Pour over the shortbread and allow to cool.
Once the caramel layer has cooled, break the Extra Dark Chocolate into pieces and place in a heatproof bowl. Place the bowl over a saucepan of gently simmering water. Allow to melt then remove from the water. Repeat this process for the White Chocolate.
Spoon the melted Extra Dark Chocolate on top of the caramel. Tap the tin on the work surface so that the chocolate covers the caramel completely. To create the marble chocolate effect blob the white chocolate on top and using a cocktail stick swirl through the Extra Dark Chocolate until the marble effect is created.
To cut, using a sharp knife, cut into 12 fingers and carefully lift from the tin. Chill until required, but stand at room temperature for a few minutes before serving.