1. Chop the chocolate into small pieces. Heat the heavy cream and pour it over the chocolate.
2. Let it sit for 2-3 minutes and stir until fully combined. Refrigerate for at least 8 hours. Put it in the mixer and whip with the whisk attachment until light and fluffy.
3. Preheat the oven to 200°C.
For the Cake:
1. In a large bowl, beat the eggs very well with the sugar and vanilla until you get a soft, fluffy mixture. Using a spatula, gently fold the flour into the egg mixture with light, large movements until combined. Be careful not to overwork the mixture, or the air will escape. Spread the mixture onto a rectangular baking tray lined with parchment paper.
2. Bake on the middle rack of the preheated oven for 8-10 minutes, or until a toothpick inserted comes out clean. Place a sheet of parchment paper on your work surface and dust it with icing sugar.
3. Carefully invert the cake onto the paper and remove the parchment paper it was baked on. Dust with a little more icing sugar and roll up lengthwise, rolling the parchment paper in with it, and let it cool.
Chestnut Mousse:
1. Whip the heavy cream with the icing sugar until it forms whipped cream.
2. Add the chestnut puree.
3. Whip again until you have a light chestnut mousse.
Assembly:
1. Unroll the now-cold roll and remove the paper. Spread 1/3 of the whipped ganache all over. Spread the chestnut mousse on top and roll up lengthwise to form a large cylinder. Cut about 1/5 of the cylinder off at a slight angle. Transfer the large piece of cake to a serving platter and turn the small piece you cut onto the middle of the large cake to create a branch.
2. Put the remaining whipped ganache into a piping bag fitted with a star tip. Pipe long, thick lines along the length of the cake, completely covering it so that it resembles tree bark. Cover each end with a little cream.
3. Dust with a little icing sugar and decorate as desired.
