An airy cocoa sponge cake rolled with cocoa-hazelnut whipped cream, cloaked in silky chocolate ganache, and finished with whimsical, wintry garnishes for a show-stopping holiday centerpiece.
Heat oven and line a 12×17-inch jelly roll pan with parchment; lightly dust a clean kitchen towel or an extra sheet of parchment with cocoa powder for rolling.
Whisk dry ingredients: cake flour, cocoa powder, baking powder, and salt; set aside.
Separate eggs. Whip egg whites with part of the granulated sugar to stiff peaks; set aside.
Whip egg yolks with remaining granulated sugar, a little oil, and vanilla until thickened and pale.
Gently fold whipped egg whites into the yolk mixture in 2 additions, then fold in the dry ingredients in 2 additions to keep the batter light and airy.
Spread batter evenly into the prepared pan and bake until set and springy (time not specified in provided text).
Immediately invert hot cake onto the cocoa-dusted towel or parchment; peel off baking parchment. Roll the warm cake up (without filling) inside the towel/parchment and let cool completely to set the shape.
Make the filling: whip heavy cream with confectioners’ sugar and cocoa powder to medium peaks; flavor with hazelnut liqueur (or alternative). Optionally fold in finely chopped hazelnuts.
Unroll cooled cake, spread the cocoa hazelnut whipped cream evenly, then re-roll into a log. Trim the ends for clean edges.
Cut a short diagonal slice from one end and attach it to the side or top of the roll to form a “branch.”
Make ganache: pour warmed heavy cream over finely chopped pure chocolate; let sit, then whisk smooth. Cool until thickened/spreadable and cover the cake. Drag a fork lightly through ganache to create a bark effect.
Decorate as desired with meringue mushrooms, sugared cranberries and rosemary, marzipan, chocolate shavings, and/or a dusting of confectioners’ sugar. Chill briefly to set before serving.