For the cake

1. Preheat your oven to 350F (175C) and line a jelly roll pan with parchment paper. Give yourself plenty of paper to hang over the edge, and fold it up in the corners to give it a bit of structure.
2. In a large, very clean mixing bowl, use an electric mixer to whisk your egg whites until they're nice and foamy. With the mixer still running, add in 1/4 cup of your sugar and continue to whisk until your have stiff peaks.
3. In another mixing bowl (I used my stand mixer for this part) whisk your egg yolks with your remaining 1/2 cup of sugar until the mixture is pale and thick. Ditch your electric mixer at this point.
4. Add the vanilla to the egg yolk mixture, then sift over the cocoa powder. Fold this all together with a rubber spatula.
5. Next, fold in your egg white in three batches. A few streaks aren't the end of the world. The egg white are all you have to leaven this cake, so don't overwork it.
6. Pour your cake batter into your waiting jelly roll pan and gently spread it evenly with an offset spatula. Bake for 20 minutes. Let the cake cool in the pan for about 10 minutes, the use the overhanging edges of your parchment paper to lift out the cake and allow it to cool completely.

For the frosting

1. Melt your chocolate by breaking it up into pieces and microwaving it for 30 seconds at a time, stirring after each go until it's completely melted and smooth. Should take about a minute. Allow this to cool for a few minutes.
2. Add your powdered sugar to the bowl of your food processor and pulse a few times to make sure you get rid of any lumps.
3. Add the butter to the food processor and process until smooth. Add the vanilla and melted chocolate and process until smooth and incorporated.

To assemble

1. Once your cake is completely cool, spread a layer of your frosting over the entire top of your cake.
2. Use your parchment paper to help you roll your cake. You want the short end facing toward you as you roll away from you. Don't panic if you have a crack or two. It will all get covered up with the frosting.
3. If you want to have a stump or two coming out of your log, slice the ends of your roll at a gentle angle. This will also give the ends of log a cleaner look. You can then use a bit of your frosting to stick this slices onto the side of your cake. Once they're covered in frosting, they'll look like stumps of branches. It's almost too easy.
4. Don't be afraid to use all of your frosting to completely cover all parts of your cake. No need to make it smooth. The rougher it is, the more it will look like bark.
5. To complete the look, us a toothpick or skewer to to make some wobbly, bark like lines down your cake. Make some rough concentric circles at each end to resemble tree rings.
6. If you want to get extra artsy, add a few cranberries and some rosemary to make a few sprigs of holly. If you want it to snow on your log, sprinkle it with some powdered sugar.