For the sponge biscuit cont... Line your pastry tray with silicone paper. Place a little of the mixture under each corner of the paper to secure it to the tray and pour in the mixture. Using a small palette knife, spread to 1cm thickness. Cook in the preheated oven for 10 minutes until a light golden colour is reached and it springs back to the touch. Remove from the oven and leave on a wire rack to cool.

For the buttercream: In a small saucepan, bring the sugar and water to 121˚C. Using a whisking machine, whisk the yolks and the 30ml of water for 5 minutes on full speed until light and aerated. Turn down to a medium speed and pour the sugar syrup halfway down on the side of the bowl, making sure it is away from the whisk; otherwise, it will simply spray the hot sugar syrup around the bowl rather than onto the egg yolks. This hot sugar syrup will partially cook the egg yolks giving you a silky, stable sabayon. Reduce the speed to medium and leave in the machine for 3 minutes to allow the mixture to cool to around 35˚C.

For the buttercream cont... On a slow speed, add the rum then gradually add the soft, room temperature butter. Whisk on full speed for 3 minutes to generate a little more lightness and volume. At this stage, you can add the chopped chestnuts. Reserve one third for filling the Bûche and the rest will be for decoration.

To build the Bûche: In a small saucepan, bring the sugar and water to the boil, then take off the heat and add the rum. Using a pastry brush, evenly soak the sponge. Place the sponge upside down onto a clean tea towel and peel away the greaseproof paper.

To build the Bûche: cont... Spread one-third of the buttercream over the soaked sponge, then roll the sponge into a tight roll 30cm long. Start by folding over the top edge towards you by about 1cm, then use this to roll the log up, making sure that it is tight all the way through. Wrap the log into a cylinder shape and refrigerate for 15 minutes. Unwrap the Bûche and place it on a serving dish or cake board. Carefully trim each end to give you a straight side. Evenly spread the remaining buttercream all over and, using a fork, mark the cream to give the impression of a log. Refrigerate for 30 minutes.

To serve: Dust with a little cocoa powder, Slice the end of the Bûche and place it in the middle on top so you can see the inside of the Bûche and give a sawn-off log effect. Garnish with a few chocolate flakes by scraping your knife down the length of your chocolate bar. Add a few whole sweetened chestnuts.

Variation: Or, if you prefer, you could change the filling to a chocolate ganache (200g chocolate melted into 200ml whipping cream) For a simple chocolate sauce to serve alongside: 2tbsp Milk 90ml Whipping cream 80g 70% Chocolate, chopped In a small saucepan on a medium saucepan, bring the milk and cream to the boil, take off the heat, and stir in the chocolate. Continue to stir until you have a smooth consistency.