Place all the ingredients for the shortbread in a food processor and whizz to a dough. Butter a 23cm square brownie tin, 4cm deep. Press the shortbread into the base of the tin – you can lay a sheet of clingfilm over the top if you wish and use your fingers to help smooth it. Prick all over with a fork, loosely cover with clingfilm and chill for at least 1 hour.
Preheat the oven to 140ºC fan oven/160ºC electric oven and bake the shortbread for 45 minutes until very lightly coloured, then leave it to cool.
Place all the ingredients for the caramel in a small non-stick saucepan and bring to the boil, stirring until melted and amalgamated. Simmer very gently for 8–9 minutes, stirring frequently and then pour over the shortbread base. Leave to cool for at least an hour until set; overnight is even better.
To make the chocolate topping, melt the dark chocolate in a bowl set over a pan with a little simmering water in it. Pour over the caramel and smooth out in a thin layer.
Melt the white chocolate in the same way in a clean bowl. To decorate, drop quarter teaspoons of white chocolate on top of the dark and marble it by swirling with a cocktail stick or skewer. Set aside in a cool place until set but still soft, then cut into squares (a small serrated knife is best for this) and leave to set completely in a cool place. If chilling, remove from the fridge 15–30 minutes before serving. This will keep well in a covered container for several days somewhere cool.
Tip: You want a fine sea salt here rather than the usual coarse one, which may not dissolve effectively.
Makes 9 – 16 large, or 25 miniature squares
Extract from Annie Bell’s Baking Bible, published by Jacana.