Heat the oven to 180°C (gas mark 4). Lightly grease an 18cm (7in) round cake tin with a little extra butter or margarine and cut a piece of greaseproof paper or non-stick baking parchment to fit the base of the tin.
Put all of the ingredients in a large mixing bowl and beat with a wooden spoon or a hand-held mixer for 1 minute, or until just combined. It's important not to beat the batter too much - just long enough to make it smooth.
Pour or spoon the mixture into the tin, smooth the top and bake on the middle shelf of the oven for about 45-50 minutes. The cake is cooked when it looks well risen and golden; the top should spring back when lightly touched with a fingertip. Another test is to insert a skewer into the centre of the cake - it should come out clean.
Let the cake sit in the tin for 5 minutes, then gently run a knife around the edge and turn the cake out onto a wire rack to cool.
For the icing, place the dark chocolate and chopped butter in a heatproof bowl and set over a saucepan of very hot water until melted. Cool for 15 minutes, then spread over the top of the cooled cake.