This traditional Christmas cake was invented in France, where it is known as 'bûche de Noël'. It's basically a chocolate Swiss roll covered in chocolate ganache and make to look like a log. Quite easy to make and very impressive to look at.
Weigh out the caster sugar and put it in a mixing bowl. Break the eggs into it and whisk them until the mixture goes pale and frothy - an electric hand whisk will do this in seconds.
Sift the flour and cocoa into the mixture and carefully fold it in with a spatula - try not to knock the air out of the eggs.
Spoon the mixture into the prepared baking tin and spread it out to form a thin, even layer.
Bake for 10 minutes. When cooked, the sponge should feel firm and have shrunk away from the side of the tin a little bit.
When the sponge is completely cool, whip the cream until it thickens. Unroll the sponge and spread the cream all over the top of it. Now roll up the sponge again, with the paper on the outside only now.
With a fork, scrape lines in the ganache to make it look a bit like bark. Sift some caster sugar over the top and sides, and decorate with pine cones, holly, plastic robins or any other small Christmas trinkets that you happen to have lying around.