Make the ganache first as it needs time to cool. Heat the double cream and butter in a pan on a low heat until the butter melts and it starts to steam, you want to to be warm but not so hot you couldn't hold your finger in it
Take it off the heat and add the dark chocolate broken into pieces. Leave for a few minutes, then stir and the chocolate should melt, keep stirring until it does. Set aside to cool
Pre-heat your oven to 160C Fan/350F/Gas Mark 4, line a large swiss roll tin or a baking tray with a lip, with baking paper. I used a Wilton Non-Stick Cookie Pan which is 33.7 x 23.5 cm (13.25 x 9.25 inches)
For the sponge, whisk the eggs and sugar together with an electric hand whisk or in a stand mixer for 8 minutes until it reaches 'ribbon stage' - this is when you can drizzle some the mix into itself in a figure 8 and it doesn't sink back immediately
Gently fold in the flour and cocoa powder in three stages, folding gently and trying not to lose too much of the volume
Pour into the tin, and spread the mixture around using a palette knife. Bake for 10 minutes, if you lightly press it in the middle with your finger tip it should spring back
Remove from the oven and tip onto a clean sheet of baking paper that has been dusted with icing sugar
Remove the baking paper the sponge was baked with, then roll up the sponge, taking the clean baking paper with it, then leave to cool completely
Whip up the double cream, icing sugar and vanilla for the filling
Gently unroll the sponge and spread the cream all over it in an even layer
Re-roll the sponge, diagonally slice about a third of the sponge off and position it along the side of the larger piece of sponge to look like a tree branch
The ganache should be thick by now, but it if is too stiff, microwave it for 15-30 seconds and stir well to loosen it up again. Pipe or spread the ganache all over the sponge. If piping, use a star nozzle to create texture. If spreading, use a fork to create texture
Dust with icing sugar and add a decorative holly leaf made from fondant in the centre if you like
Serve immediately, store any leftovers in the fridge for 2-3 days