Photo: Clare Winfield
Preheat the oven to 180°C/350°F/Gas mark 4. Grease a 20 x 30cm / 8 x 12in baking tray or Swiss roll tin and line with baking parchment.
Whisk the egg yolks and caster sugar together for a couple of minutes until pale and fluffy. Sift in the cocoa powder and whisk until it comes together.
In a spotlessly clean bowl, whisk the egg whites and salt until soft peaks form. Carefully fold this into the chocolate mixture, then just as carefully scrape into the baking tray. Bake for 20–25 minutes until springy to the touch. Leave on the tin to cool for 20 minutes or so.
Dust a large piece of baking parchment with icing sugar, and carefully turn the chocolate base onto this. Remove the top layer of parchment.
For the filling, whip the cream until stiff. Fold through the chestnut purée and the brandy. Spread this evenly over the chocolate base. Using the parchment, roll the base lengthways into a long log.
Transfer to a large plate or board. If it cracks, panic not. Just pat it together for now. Cut off one end on a diagonal and reposition to form a branch (see photograph). Cover and chill.
For the icing, put the cream in a saucepan over a gentle heat and bring to just short of a boil, then take off the heat. Add the chocolate and leave for a few minutes, then beat with a spoon until smooth and glossy. Leave to cool, then spread all over the chocolate log. Use a fork to make bark-like marks. Cover and chill in the fridge.
To serve, dust with icing sugar and cut into fat slices. With more cream if you’re feeling particularly decadent.