Melt 100 g (3½ oz) of the chocolate and the butter in a heatproof bowl set over a saucepan of gently simmering water (don't let the base of the bowl touch the water).
Beat the eggs and sugar in a separate bowl until light and foamy. Stir in the melted chocolate mixture. Tip in the flour, remaining chopped chocolate and the nuts and mix together until just combined.
Spoon the mixture into a greased and lined 28 x 18 cm (11 x 7 inch) shallow baking tin or roasting tin (see