Preheat the oven: Set your oven to 375°F (190°C) and line a jelly roll pan (15x10 inches) with parchment paper.

Prepare the egg mixture: Beat the egg yolks and 1/2 cup of sugar until pale and thick. Mix in the vanilla extract. In a separate bowl, whisk the egg whites, cream of tartar, and remaining sugar until stiff peaks form.

Combine dry ingredients: Sift together the flour, cocoa powder, and salt.

Fold the batter: Gently fold the dry ingredients into the yolk mixture, followed by the whipped egg whites. Be careful not to deflate the batter.

Bake the cake: Spread the batter evenly on the prepared pan and bake for 12-15 minutes or until the cake springs back when touched.

Roll the cake: While warm, invert the cake onto a clean kitchen towel dusted with powdered sugar. Peel off the parchment paper and roll the cake tightly with the towel. Let it cool completely.

Whip the cream: Beat heavy cream, powdered sugar, and vanilla extract until stiff peaks form.

Unroll and fill: Carefully unroll the cooled sponge cake and spread the whipped cream filling evenly. Reroll the cake without the towel.

Heat the cream: Warm the heavy cream until it begins to simmer. Pour it over the chopped chocolate and butter. Stir until smooth and glossy.

Frost the cake: Spread the ganache over the rolled cake, using a fork to create a bark-like texture.

Garnish: Dust the cake with powdered sugar and add fresh cranberries and rosemary sprigs for a festive touch.