300g/10½oz dark chocolate (around 35-40% cocoa solids), broken into small pieces
For the cream filling
300ml/½ pint double cream, whipped
Icing sugar, for dusting
A toy snow man or sprig of holly
Preheat the oven to 200C/400F/Gas 6. Lightly grease a 33x23cm/13x9in Swiss roll tin, and line with non-stick paper or baking parchment.
For the sponge, in a large bowl whisk the eggs and sugar using an electric hand whisk until the mixture is pale in colour, light and frothy. Sift the flour and cocoa powder into the bowl, cut and fold together until all the cocoa and flour are incorporated into the egg mixture.
Pour the mixture into the lined tin and spread evenly out into the corners. Bake in the middle of the preheated oven for 8–10 minutes, or until well risen and firm to the touch and the sides are shrinking away from the edge of the tin.
Place a sheet of baking parchment on the work surface, dust with icing sugar generously, invert the cake onto the paper, remove the bottom lining piece of paper.
Begin to tightly roll up the sponge using the paper. Roll with the paper inside and sit the roll on top of its outside edge to cool completely.
To make the ganache topping, finger-warm the cream in a pan, remove from the heat and add the chocolate stirring until it melts. Put into the fridge to thicken up.
Uncurl the cold Swiss roll, remove the paper. Spread the whipped cream on top and re-roll tightly. Cut a quarter of the cake off from the end on the diagonal. Transfer the large piece of cake to a serving plate and angle the cut end in to the middle of the large cake to make a branch.
Put the chocolate icing into a piping bag fitted with a star nozzle. Pipe long thick lines along the cake, covering the cake completely so it looks like the bark of a tree. Cover each end with icing or, if you wish to see the cream, leave un-iced..
Dust with icing sugar and garnish with fresh holly or a little robin to serve