Lightly grease a 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 the ingredients into a large mixing bowl and beat with a wooden spoon or a hand-held mixer for 1 minute. You can also mix in a food processor. It's important not to beat the batter too much - just long enough to make it smooth.
Pour 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 or a clean knife into the center of the cake - it should come out clean
Let the cake sit in the tin for 20 minutes, to cool