Sebbie's Chocolate Cake

1. Heat the oven to 180C (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.
2. Put all 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.
3. 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.
4. 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.
5. 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.