100²µ/3½´Ç³ú caramel
130²µ/4¾´Ç³ú plain flour
280g/10oz caster sugar
20²µ/¾´Ç³ú cocoa powder
pinch rock salt
250g/9oz dark chocolate, minimum 70% cocoa solids, broken up or roughly chopped
100²µ/3½´Ç³ú white chocolate, finely chopped
250g/9oz salted butter, plus extra for greasing
4 free-range eggs