Preheat your oven to 350°F (175°C). Whip the egg whites in a clean bowl until soft peaks form, then set aside. In another bowl, beat together yolks, sugar, and vanilla until pale. Combine the dry ingredients and add them to the yolk mixture, then gently fold in whites in two stages so you don’t deflate too much.
Pour the batter into the prepared parchment-lined pan, spreading evenly, and bake for 10–12 minutes until the cake is springy and not wet. Immediately turn it out onto a powdered sugar–dusted kitchen towel, peel off parchment, then roll it (with the towel) from one short end. Let it cool completely.
While sponge bakes, melt together chocolate, butter, vanilla, salt, and cream for ganache. Mix whipped cream with sugar and vanilla until stiff peaks. Unroll the cooled cake, spread whipped cream (leaving small edges free), then re-roll and place seam side down. Trim one end at an angle to act as a branch, then coat the entire cake with ganache. Use a fork or skewer to draw bark lines. Garnish with berries, rosemary, and a dusting of powdered sugar.