To begin, line a 18 x 28cm (7 x 11in) Swiss-roll tin with baking parchment and grease well. Do this, even if you're using a non-stick Swiss-roll tin.
Pre-heat your oven to Gas 4/180°C/350°F.
Beat the eggs and sugar until pale and fluffy. They will increase in volume, so make sure you use a large bowl. You're done when you lift your whisk out and it leaves a ribbon trail.
Now gently fold in the flour and cocoa powder. Use a big spoon and work quickly. You don't want to knock out too much of the air that you've just worked so hard to put in.
Pour the mixture into the prepared tin and spread out.
Bake for approx. 20-25 mins. It's done when the centre of the cake springs back when lightly pressed.
Remove the sponge from the oven and turn out onto another sheet of greaseproof paper that you've sprinkled with sugar. Leaving both sheets of paper in place, gently roll up the sponge. Place somewhere safe and leave until completely cooled.
Meanwhile, make the filling.
For the chocolate filling you can either make chocolate butter icing or mix a thick chocolate cream. Choose whichever you like better.
Chocolate Butter Icing
This is easy to make. Just mix
75g (3oz) butter
150g (6oz) sifted icing sugar

2 tsp cocoa powder
1-2 tbsp water

together until light and fluffy.
Chocolate Cream
Somehow, this comes out lighter than the butter cream mixture. Make sure you use really good dark chocolate (at least 70% cocoa solids) for the cream. Cheap chocolate does nothing for your yule log.
You need:
284ml carton whipping cream

200g (8oz) best dark chocolate

Break up the chocolate into chunks and melt over a bowl of hot (not boiling!) water. Make sure the water does not touch the bowl or the chocolate will curdle.
In a small pan heat the cream. When close to boiling point, take off the heat and stir gently into the melted chocolate until completely combined.
Leave to cool, then place in the fridge to chill.
When ready to use (and very cold) beat the mixture until fluffy. Go careful here, you don't want to end up with chocolate butter!
Assembling the Yule Log
When ready to assemble, carefully unroll your sponge and peel off the greaseproof backing paper.
Spread about 1/4 of the filling over the sponge, then roll up carefully with the help of the second, sugar-covered paper.
Place on a large plate or tray, join underneath.
Cover with the rest of the chocolate cream. You can do this quite roughly with a spatula. Alternatively, use a fork to mark the covering into the resemblance of tree bark.
Dust with a little cocoa powder or sprinkle dark chocolate curls and shavings over the top.
Decorate with holly leaves and berries.g