1-Make the chocolate buttercream

• Melt the dark chocolate in a heatproof bowl over a pan of simmering water (bain-marie) or in the microwave in 20–30 second bursts, stirring each time. Leave to cool for about 5 minutes until just warm.

• Place the soft butter in a large bowl and beat for 2–3 minutes until creamy, smooth and slightly paler in colour.

• Add the icing sugar (and cocoa if using) gradually, beating on medium speed until you get a thick, smooth cream with no lumps.

• Pour the cooled, melted chocolate in a thin stream into the butter mixture while beating. Continue until the buttercream is glossy, even and nicely chocolatey.

• If the buttercream is too thick, add 1–2 tbsp of milk or cream, a little at a time. If it’s too soft, beat in a bit more sifted icing sugar.

• Place the bowl in the fridge for 10–15 minutes so the buttercream firms up slightly and is easier to spread on the sponge.

2. Bake the sponge roll

• Line a 25 x 40 cm (10 x 16 in) baking tray with lightly buttered baking paper. Preheat the oven to 180°C (350°F), conventional heat.

• Sift the flour and baking powder together into a bowl to get rid of any lumps.

• In a large mixing bowl, whisk the eggs, sugar and vanilla until the mixture is very thick, pale and mousse-like. Using a stand mixer, use the whisk attachment; with a hand mixer, tilt the whisk slightly to incorporate more air. Beat for 8–10 minutes, until the volume has tripled.

• Pour in the vinegar and whisk for a few more seconds. It helps the sponge stay supple and easy to roll.

• Add the flour mixture one spoonful at a time, gently folding with a spatula using upward, sweeping motions. Take your time and avoid knocking out the air.

• Pour the batter into the prepared tray and spread it out evenly with a spatula so the thickness is uniform.

• Bake in the preheated oven for 10–12 minutes, until lightly golden and springy to the touch. Do not overbake or the sponge may crack when rolled.

3. Fill, roll and chill the Yule log

• As soon as the sponge comes out of the oven, turn it out onto a clean tea towel. Carefully peel off the baking paper. Roll the sponge up immediately, starting from the short side, with the tea towel inside. Leave to cool completely in this rolled shape.

• Once the sponge is cool, gently unroll it. Trim the edges with a serrated bread knife to neaten them. Spread a generous, even layer of chocolate buttercream over the entire surface, leaving a small border at one end.Roll the sponge up again, this time without the tea towel, as tightly as you comfortably can. Wrap the log in aluminium foil (or cling film) and place in the fridge for at least 30 minutes to set the shape.

• Remove the Yule log from the fridge, unwrap it and trim the ends for a clean look. Cover the outside with the remaining chocolate buttercream. You can create a “tree bark” effect by dragging a fork along the icing, then decorate with chocolate shavings, icing sugar “snow” or festive decorations.