Chop 150 g (5 oz) of the chocolate into 5 mm (¼ inch) pieces. Break the remaining chocolate into pieces and put in a heatproof bowl with the butter. Melt 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 and chopped chocolate 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