Beat together the butter, sugar, eggs, flour and cocoa powder in a bowl until smooth and creamy. Add 125 g (4 oz) of the chopped chocolate to the bowl. Mix well.
Spoon the mixture into a greased and lined 500 g (1 lb) or 750 ml (1¼ pint) loaf tin (see
Make the buttercream. Melt the chocolate in a heatproof bowl set over a saucepan of gently simmering water (don't let the base of the bowl touch the water). Meanwhile, beat together the butter and icing sugar in a bowl until pale and creamy. Beat in the melted chocolate. Spread over the top of the cake and scatter with the reserved chopped chocolate.