Take a 33x23cm Swiss roll tin or baking tray and cut a double layer of non-stick baking parchment slightly bigger than the tin. Lay the parchment in the tin. Don’t worry if the edges stick up untidily round the sides.
Preheat oven to 200C/400F/Gas 6.
Put the chocolate and coffee into a heavy-based saucepan with 85ml/3fl oz water and put over a low heat to melt the chocolate.
Beat the egg yolks and the caster sugar until pale and mousse-like. Add the melted chocolate.
Whisk the egg whites until stiff but not dry. With a large metal spoon, stir a small amount thoroughly into the chocolate mixture to loosen it. Fold the remaining whites in gently. Spread the mixture evenly in the lined roasting tin.
Bake in the preheated oven for about 12 minutes, or until the top is slightly browned and firm to touch.
Slide the cake parchment out of the roasting pan onto a wire rack. Leave to cool completely (do not cover).
To make the filling, whip the cream in a bowl, then mix in the sweetened chestnut purée.
When the sponge base is cooled all the way through (check underneath) you are ready to assemble the roulade. Put greaseproof paper on a work surface. Turn out the roulade onto the paper. Carefully remove the paper used to line the tin. Spread the filling evenly across the surface of the cake.
Using the paper under the cake to help, roll it up firmly from one of the narrow ends. Rest the cake on its seam. Wrap tightly with the greaseproof paper and refrigerate for at least 30 minutes and up to overnight. (It doesn't matter if the cake breaks apart when rolling - a little sifted icing sugar will do wonders for its appearance.)
Serve dusted with icing sugar and garnish with holly leaves and berries.
To help assemble the roulade without cracking it is important you do not cover the cake with a tea-towel when cooling or pre-roll the cake before adding the filling (as you might do with a normal Swiss roll).
If this cake is to be used as a Yule log the tendency to crack is a positive advantage: the firm skin will crack very like the bark of a tree. Sprigs of holly or marzipan toadstools help give a festive look, and a dusting of icing sugar looks like snow.