Preheat the oven to 200°C(180°C fan assisted) and line a swiss roll tray with paper. Whisk together the sugar and eggs for 5-10 minutes until it forms a pale, creamy mixture. The whisk should leave trails.
Sieve in the flour and cocoa powder and mix well. Be careful not to knock the air out but make sure it is well combined with no clumps.
Spread the mixture out evenly into the prepared pan. Using a palette knife to ease it into the corners. Place in the centre of the pre-heated oven for 10 minutes (or until the cake is springy to the touch. I could have baked mine for a minute or so less).
Remove from the oven and allow to cool for a few minutes in the pan, covered with a damp cloth. In the meantime, lay out a large piece of greaseproof paper and dust with cocoa powder. Transfer the cake to this.
While it cools on the new parchment, make the buttercream. Melt the chocolate, either using a heat proof bowl over a saucepan of water, or in the microwave. Set aside and allow to return to room temperature.
Sift the icing sugar in a mixer with the butter and cream together. Pour in the chocolate and cream until well mixed.
Spread the buttercream over the whole cake, making sure it goes to all the corners. Carefully roll the swiss roll from the longest edge close to you, getting the roll as tight as possible.
Once rolled tightly, cut off either end at a slight angle, use these to make the branches of your log.
Cover the log in the remaining buttercream, using a fork afterwards to drawn the pattern of the bark. Dust with Icing Sugar to get it looking extra festive.
Recipe by The Bearded Bakery at http://www.thebeardedbakery.com/baking/cakes/chocolate-yule-log/