Grease a 20 x 30 cm (8 x 12 inch) baking tin and line the base with baking paper. Put the chocolate and butter in a heatproof bowl set over a saucepan of gently simmering water (don't let the bowl touch the water) and stir over a low heat until melted. Leave to cool for 5 minutes.
Meanwhile, using an electric hand-held whisk, whisk together the eggs and sugar in a bowl for 5 minutes until thick. Beat in the cooled chocolate mixture and fold in the flour.
Spoon the mixture into the prepared tin and bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 45–50 minutes until risen and firm to the touch. Leave to cool in the tin for 10 minutes, then turn out on to a wire rack to cool completely, removing the paper from the base.
Meanwhile, make the icing. Put the chocolate in a saucepan with the cream and heat gently, stirring, until the chocolate has melted. Leave to cool for 1 hour until thickened to a pouring consistency, then spread over the cake. Leave to set for 30 minutes before serving.