To make the sponge cake: preheat the oven to 190°C.
Separate the egg white from the egg yolk.
Beat the egg yolk until they are thick and have doubled in volume. Add half of the sugar and whisk vigorously to obtain a firm meringue mixture.
Beat the egg whites with the rest of the sugar and then add the warm milk. Sift in the flour and the cocoa powder. Add the groundnut oil and mix with a spatula. Incorporate the meringue and then spread the mixture on to a baking sheet. Transfer to the oven and bake for 15 minutes. Once cooked, wrap the sponge cake in cling film to keep in the moisture.
For the syrup: place the water, rum and caster sugar in a saucepan and bring to the boil. Set aside.
For the ganache: whip 200ml cream until thick. Break the chocolate in to pieces. Bring the rest of the cream to the boil then pour over the chocolate pieces. Mix until smooth then delicately fold in the whipped cream.
To assemble: remove the cling film from the sponge cake. Pour the syrup over the sponge cake. Spread ganache over the chocolate sponge (keeping back enough ganache to cover the yule log) and then roll up to make the yule log. Spread the rest of the ganache over the log and then decorate with icing sugar, cocoa powder and some pieces of chocolate if desired.
«Decorate the yule log with a few pieces of marzipan if desired.»