Put the cocoa powder in a heatproof bowl and whisk in the measurement water. Tip in 125 g (4 oz) of the chocolate and leave to cool, stirring frequently to melt the chocolate.
Beat together the butter, sugar, eggs, flour, bicarbonate of soda and vanilla in a bowl until pale and creamy. Stir in the chocolate mixture and remaining diced chocolate.
Spoon the mixture into a greased and lined 1.25 kg (2½ lb) or 1.5 litre (2½ pint) loaf tin (see
Make the topping. Put all the ingredients in a small saucepan and heat very gently until melted. Remove from the heat and leave until the chocolate has melted, stirring frequently. Leave to cool slightly, then spread over the cake.