Chocolate Swiss Roll (Bûche de Noël)

Whisk the eggs to a foam. Heat the oven to 200°C (180°C fan) for at least 20 minutes and line the Swiss roll tin with baking paper. Whisk the eggs, sugar and salt on high speed for 6-8 minutes until the mixture has roughly tripled in volume and holds a ribbon trail for about 5 seconds when the whisk is lifted. There's no other raising agent in this sponge, so all of its lift comes from the air beaten in at this stage.

Fold in flour and cocoa. Sift the flour and cocoa powder over the whisked eggs in two additions, folding each in gently with a large metal spoon rather than stirring. Spread the batter evenly out to the corners of the tin with a palette knife. Gentle folding keeps the air bubbles intact, which is what gives the baked sponge enough flexibility to roll without tearing.

Bake and roll while hot. Bake for 8-10 minutes until the sponge feels just springy; a minute or two too long and it dries out and cracks when rolled. Turn the sponge out immediately onto a sheet of baking paper dusted with sugar, peel away the lining paper, then roll it up from a short end with the fresh paper inside and wrap it in a tea towel to cool completely. Rolling it while still warm trains the sponge into a curl, so it won't crack later when it's unrolled for filling.

Make the ganache bark. Bring the cream and golden syrup to the boil, pour it over the chopped chocolate, leave for 2 minutes, then stir until smooth and glossy. Leave the ganache at room temperature for 1-1½ hours until it thickens to a spreadable consistency. The syrup keeps the set slightly soft, which is what lets it spread thinly like bark instead of setting hard.

Fill and re-roll the sponge. Whip the double cream with the icing sugar and vanilla to soft-firm peaks. Unroll the cooled sponge, spread the cream over it evenly, leaving a 2 cm bare strip at the far end, then roll it back up without the paper, finishing with the seam underneath. That bare strip stops cream from squeezing out of the end as the roll tightens. Chill the filled log for 30 minutes to firm up.

Shape and bark the log. Slice a 6 cm piece off one end of the log at an angle and set it against the side of the log to look like a branch. Cover the whole log and branch with the ganache using a palette knife. Drag a fork lengthways along the ganache to score ridges that look like bark. Chill for 1 hour until firm, then dust with icing sugar just before serving.