Prepare your baking surface and oven. Preheat oven to 350°F (175°C). Line a 15×10-inch jelly roll pan with parchment paper, leaving an overhang on all sides. Grease the parchment paper lightly.
Beat egg whites until foamy. In a large bowl, beat egg whites with an electric mixer until soft peaks form. Gradually add 1/4 cup of the sugar and continue beating until stiff, glossy peaks form.
Prepare the egg yolk mixture. In another bowl, beat egg yolks with remaining 1/2 cup sugar until pale and thick (about 3-4 minutes). Mix in vanilla extract.
Combine the dry ingredients. Sift together cocoa powder, flour, and salt. Gently fold this mixture into the egg yolk mixture.
Incorporate the egg whites carefully. Fold about 1/3 of the beaten egg whites into the chocolate batter to lighten it, then gently fold in the remaining whites, taking care not to deflate the mixture.
Bake the sponge cake. Spread the batter evenly in the prepared pan and bake for 12-15 minutes, until the cake springs back when touched lightly.
Prepare for rolling while hot. Dust a clean kitchen towel generously with powdered sugar. When the cake comes out of the oven, immediately invert it onto the towel. Carefully peel off parchment paper.
Roll the cake while warm. Starting from the short end, roll the cake up with the towel inside. Place seam-side down on a cooling rack and let cool completely (about 1 hour).
Make the chocolate buttercream. Beat butter until creamy. Gradually add sifted powdered sugar and cocoa powder, alternating with heavy cream. Beat in vanilla and salt until smooth and fluffy.
Unroll and fill the cake. Gently unroll the cooled cake and remove the towel. Spread about 2/3 of the buttercream evenly over the cake, leaving a 1/2-inch border.
Re-roll and chill the log. Carefully roll the cake back up (without the towel). Place seam-side down on a serving platter and refrigerate for 30 minutes.
Create the branch effect. Cut a diagonal slice from one end of the log (about 2 inches). Attach this piece to the side of the log using buttercream to create a branch.
Frost and decorate. Cover the entire log with remaining buttercream, using a fork to create bark-like texture. Dust lightly with cocoa powder and powdered sugar to create a realistic woodland effect. Add sugared cranberries and rosemary sprigs if desired.
Chill before serving. Refrigerate for at least 1 hour or up to 24 hours before serving to allow flavors to meld and frosting to set.