Preheat oven to 190 °C (≈ 375 °F). Butter and line a Swiss-roll tin with parchment paper. :contentReference[oaicite:2]{index=2}

Separate the eggs. In one bowl, whisk the egg yolks with sugar and vanilla for several minutes until pale and thick. In another bowl, beat the egg whites with a pinch of salt until stiff peaks form. :contentReference[oaicite:3]{index=3}

Sift together the flour and cocoa powder. Gently fold the dry mixture into the yolk mixture, then carefully fold in the beaten egg whites in batches to keep the batter light and airy. :contentReference[oaicite:4]{index=4}

Pour the batter evenly into the prepared tin and bake 10–12 min (or until the cake springs back lightly). Remove and immediately turn the cake out onto a clean tea towel dusted with powdered sugar. Peel off the parchment and roll the cake (with the towel) from the short end. Allow to cool completely. :contentReference[oaicite:5]{index=5}

Meanwhile, prepare the chocolate ganache: bring the heavy cream just to a simmer, pour over the chopped chocolate (and butter if using), let sit a minute, then stir until smooth and glossy. Let it cool until spreadable. :contentReference[oaicite:6]{index=6}

Carefully unroll the cooled cake. Spread most of the ganache inside (or alternatively a whipped cream or buttercream filling), leaving a small margin at the edges. Re-roll the cake without the towel. Place seam-side down on a serving plate. :contentReference[oaicite:7]{index=7}

Use the remaining ganache to frost the outside of the log. With a fork or spatula, mark lines or “bark-like” texture on the frosting to mimic wood grain. :contentReference[oaicite:8]{index=8}

Optionally, decorate with powdered sugar (for snow effect), meringue or marzipan mushrooms, sprigs of rosemary/holly for a “woodland” look. Chill until serving. :contentReference[oaicite:9]{index=9}