For the chocolate sponge, whisk the eggs and sugar together until the mixture thickens slightly and a ribbon of mixture falls from the whisk (ribbon stage). Sieve the cocoa powder and flour together, and then fold the dry ingredients in to the egg and sugar mix. Spread the mixture very carefully onto baking parchment, place onto a baking tray and bake at 180 degrees for 6-8 minutes.
Turn the sponge out onto sugared parchment paper and peel off the parchment paper the sponge was cooked on. Roll the sponge up in the sugared parchment paper and allow to cool.
For the chocolate mousse, firstly whisk the cream to nearly ribbon stage. Place the ganache in a saucepan and heat on a very low temperature until the ganache has melted to 37 degrees. Fold the whisked cream and heated ganache together. Take the cool chocolate sponge, spread the mousse through the middle and roll up tightly to form a swiss roll. Coat the outside of the swiss roll with the remaining mousse and allow to
set.
To decorate the yule log, coat with the ganache, garnish with the marzipan plaque, toadstools and dust with icing sugar.