3ozHigh-quality dark chocolate85g, as chips or broken into small pieces
Preheat the oven to 350°F/175°C or 160°C if you have a convection oven
Melt the butter and pour it into the jelly pan. Use a brush to coat it well, including the corners and pour the excess butter out into a dish. Next line the pan with baking paper and then brush the top of the paper with the remaining butter. Get every inch of it.
Whisk the dry ingredients for the sponge together in a bowl. Set aside.
Pour the yolks into your electric mixer's bowl. Add the vanilla and ¼ cup of the sugar and mix on medium speed for a few minutes or until pale yellow.
Take your mixer's beaters off and rinse thoroughly. In a second bowl that will fit with your electric mixer, add the egg whites and begin beating at medium-high speed. When they begin forming peaks, add the remaining ½ cup of sugar a spoonful at a time. The peaks will look glossier after and the entire step only takes a few minutes.
With a rubber spatula, gently fold the egg yolk mixture into the egg whites. Next, sprinkle over the dry ingredients and fold those in too. Mix until incorporated but don't over-mix. You want the air from the egg whites to stay in the batter.
Pour the batter into the pan and smooth with your spatula. Bake for 8-10 minutes or until the cake springs back a little when touched. Take the cake out of the oven and cool.
When cooled to room temperature, make the filling. Whip the cream, sour cream, and remaining three tablespoons of powdered sugar in the mixer until it forms stiff peaks.
Spread this cream evenly over the top of the cake sponge, except for a couple of inches on one of the short sides.
Rolling the cake is pretty easy at this point, don't worry. Begin from the short end that you did cover with cream and lift up the edge. Begin rolling it towards the other end, peeling off the paper as you go. If the cake cracks a little, don't worry since the roll will hold it in place. Roll all the way to the un-creamed side then lift the cake up and place it seam-side down on the serving dish.
Time to make the chocolate ganache. Place the cream in a saucepan and bring to a boil. Take it off the heat, add the chocolate and let it sit together for five minutes. While it's sitting, place baking paper all around the cake and tuck it just under its edge. This helps with chocolate spillage.
Stir the chocolate and cream together and then let it cool slightly. About ten minutes. Next, slowly pour it over the yule log so that the cake is covered. You'll see what I mean about the paper helping with the spillage here. Wait another five minutes before running a fork along the ganache to create tree bark texture.
You can leave this cake at room temperature if you're planning on serving it the same day. If you're making it the day before, refrigerate it and take it out to come to room temperature before serving.