Begin by preheating your oven to 375°F (190°C) and line a jelly roll pan with parchment paper, greasing lightly.

In a large bowl, whisk together sugar and egg yolks for about 5 minutes until pale. Beat egg whites in a separate bowl until stiff peaks form, then fold into yolk mixture with sifted flour and cocoa.

Spread the batter in the prepared pan and bake for 12-15 minutes until it springs back lightly when touched.

Dust a clean kitchen towel with powdered sugar, invert the warm cake onto it and roll it up tightly. Let cool completely in this rolled shape for about 30 minutes.

Whip the chilled heavy cream until stiff peaks form, then gradually add powdered sugar and vanilla.

Unroll the cooled cake, spread the whipped cream filling, then re-roll it tightly and chill seam-side down for at least 1 hour.

Beat softened butter until creamy, then mix in powdered sugar, cocoa powder, and enough milk for spreadable consistency.

Spread the buttercream over the log, use a fork to create bark-like texture, sprinkle with powdered sugar, and decorate with cranberries and rosemary.