Bûche de noël

Method

• step 1

  Heat the oven to 180C/160C fan/gas 4. Butter a 32cm x 23cm swiss roll tin and line with baking paper. For the sponge, beat the egg whites in a large bowl using an electric whisk to stiff peaks. Whisk in 2 tbsp of the caster sugar.

• step 2

  In a separate bowl, whisk together the egg yolks and remaining caster sugar for 5 mins until pale and fluffy, and the whisk leaves a three-second ribbon in the mixture when lifted out. Sift in the cocoa powder and whisk until evenly combined. Add a third of the whipped egg whites to the chocolate mixture, stirring to combine and loosen the batter, then, using a large metal spoon, carefully fold in the remaining egg whites in a figure-of-eight motion so as not to knock out the air. Pour the mixture into the prepared tin, smoothing it using the back of a spoon. Bake on the middle shelf of the oven for 20-25 mins or until risen and a skewer inserted into the middle comes out cleanly. Cool in the tin for 5 mins.

• step 3

  Lightly dust a large sheet of baking paper with icing sugar, then invert the sponge onto it, cover with a clean tea towel and leave to cool fully. Once cool, carefully remove the tea towel and baking paper.

• step 4

  Meanwhile, make the filling. Whisk together the cream, sugar and vanilla bean paste until thickened but still spreadable.

• step 5

  For the topping, beat the butter until smooth, then whisk in the icing sugar and cocoa. Melt the chocolate in a heatproof bowl over a pan of simmering water (making sure the bowl doesn't touch the water) or the microwave in 20-second bursts. Cool slightly, then whisk the melted chocolate into the buttercream until smooth. Stir in 1 tbsp of boiling water from the kettle to loosen the buttercream, if needed.

• step 6

  Spread the vanilla cream filling over the sponge, leaving a 2cm border around the edge. Lift the longest edge of the cake up and over the filling to roll into a tight log, using the baking paper to help you. Transfer to a serving platter, then cut off the bottom quarter of the roulade on a diagonal and arrange it on the side so it looks like a branch on the log. Smooth the buttercream over the roulade using a palette knife, then use a fork to create lines for a wood-grain effect. Dust with icing sugar and decorate with redcurrants or fresh cranberries and holly or rosemary, if you like. Keeps in the fridge for up to three days.