@Redwinestillfine, I'd hardly call it a recipe, it's dead easy!
Just make a simple chocolate swiss roll mixture - there are loads of recipes online, such as the BBC good food one. Whisk the eggs for at least 8 mins to get a nice light sponge.
When it comes out of the oven in its flat tray, spread out a clean hot damp teatowel, with a sheet of greaseproof paper flat on top. Turn out the swiss roll onto the paper, then quickly roll it up with the paper so the paoer is inside each layer of the roll and round the outside too. (but not the wet tea towel, obvs!)
Drape the damp towel over the paper bundle to keep it moist, and leave it to cool.
Meanwhile, in a bowl whisk up some double cream until its really thick. Mix in a packet of the chestnut puree (Merchant Gourmet make a perfect size packet) and enough icing sugar to sweeten it to your taste.
When the roll is completely cool, gently unroll it, discard the paper, spread it generously with the cream and chestnut mix, then roll back up again (but without paper this time).
Next, melt some Nutella in the microwave just long enough to make it really runny.
Pour over the outside of the swiss roll, and if you like, fork it a little to roughen the surface and look more log like.
When it has set, and just before serving, sieve a tiny sprinkle of icing sugar over it to look like snow. I always pick a holly sprig from the garden, wash it, then wrap the stem in a little tin foil to avoid germs, and stick it lightly into the top of the log as decoration.
I make the log a few weeks ahead and freeze it, so I just have to decorate it on Xmas Eve, when time is short.