- 4 free-range eggs, separated
- 60g/2¼oz caster sugar
- 60g/2¼oz plain flour
- 15g/½oz unsalted butter, melted
- 70g/2¾oz caster sugar
- 3 free-range egg yolks
- 30ml/1¼fl oz dark rum
- 240g/8¾oz unsalted butter, softened to room temperature
- 200g/7oz candied chestnuts, half reserved whole, the remainder roughly chopped
- 40g/1¼oz caster sugar
- 25ml/1fl oz dark rum
- 2 tbsp milk
- 90ml/3fl oz whipping cream
- 80g/2¾oz plain chocolate, minimum 70 per cent cocoa solids, chopped, plus extra grated chocolate, to decorate
- cocoa powder, to decorate
- For the genoise, preheat the oven to 180C/160C Fan/Gas 4.
- Whisk the egg whites in a bowl (or free-standing mixer) until soft peaks form when the whisk is removed. Gradually add the sugar as you whisk the egg whites, until stiff peaks form when the whisk is removed.
- Reduce the speed of the electric whisk to medium, then whisk the egg yolks into the mixture, one by one.
- Sift half of the flour into the bowl, then gently fold it into the egg mixture using a spatula. Add the remaining flour and carefully fold it into the egg whites too.
- Pour over the melted butter and gently fold it into the egg white mixture.
- Line a 30cm/12in Swiss roll tin with silicone paper, sticking each corner to the tin with a dab of the genoise mixture.
- Pour the genoise into the prepared tin, pushing it into the corners and levelling the surface using a palette knife.
- Bake the genoise in the oven for 10 minutes, or until it is pale golden-brown in colour and springy to the touch. Remove from the oven and set aside to cool in the tin.
- Meanwhile, for the buttercream, heat the sugar and 50ml/2fl oz water in a small saucepan until the temperature of this sugar syrup reaches 121C (use a sugar thermometer for this).
- Using an electric whisk or free-standing mixer, whisk the egg yolks with 30ml/1fl oz water for 5 minutes on full speed until pale and fluffy.
- Reduce the speed of the electric beaters, then drizzle the sugar syrup in a thin stream down the side of the bowl. (Caution: do not pour the hot syrup directly onto the beaters). Whisk until the mixture thickens and looks silky, then continue to whisk until the mixture cools completely.
- Reduce the speed of the beaters to a slow speed, then whisk in the rum. Add the softened butter a little at a time, whisking until each addition has become fully incorporated into the sabayon, then continue to whisk for a further 3 minutes.
- Fold in the chopped chestnuts and set aside until needed (chill in the fridge if making in advance).
- For the soaking liquid, bring the sugar and 40ml/1¼fl oz water to the boil in a small saucepan, then remove the pan from the heat and stir in the rum.
- Using a pastry brush, brush the soaking liquid evenly over the cooled sponge.
- Turn the soaked sponge upside down onto a clean tea towel, then carefully peel away the silicone paper.
- Spread one-third of the buttercream in an even layer all over the soaked sponge. Place the candied chestnuts in a line where the inside of the roll will be. Then roll the sponge as tightly as possible using the tea towel to lift the sponge.
- Wrap the sponge roll tightly in cling film, then chill in the fridge for 15 minutes.
- Unwrap the sponge roll and transfer it to a cake board with the seam on its underside. Carefully trim each end using a sharp knife.
- Spread the remaining buttercream all over the sponge roll in an even layer using a palette knife (or piping bag, if desired). Drag a fork lightly and unevenly through the buttercream to make it look like tree bark. Refrigerate for a further 30 minutes.
- Meanwhile, for the chocolate sauce, bring the milk and cream just to the boil in a saucepan. Remove the pan from the heat and stir in the chocolate until smooth and melted.
- Just before serving, dust the bûche de Noël with a little cocoa powder and garnish with grated chocolate or chocolate curls. If desired, slice the end off the log and place it on top.
- To serve, slice the bûche de Noël and drizzle with a little of the chocolate sauce.