Step 1: Prepare the Sponge Cake Batter: Preheat oven to 350F (175C). Line a 10x15 inch jelly roll pan with parchment paper, leaving an overhang on the sides. In a large bowl, beat egg yolks with cup of the granulated sugar and vanilla extract until pale and thick. In a separate clean bowl, beat egg whites with salt until soft peaks form. Gradually add the remaining cup of granulated sugar and continue beating until stiff, glossy peaks form.
Step 2: Combine Wet and Dry Ingredients: Gently fold the egg yolk mixture into the egg white mixture until just combined. Sift together the flour and cocoa powder, then gently fold into the egg mixture until just combined. Be careful not to overmix.
Step 3: Bake the Sponge Cake: Spread the batter evenly into the prepared jelly roll pan. Bake for 12-15 minutes, or until a toothpick inserted into the center comes out clean.
Step 4: Roll the Cake: While the cake is baking, lay a clean kitchen towel on a flat surface and dust it generously with powdered sugar. Once the cake is done, immediately invert it onto the prepared towel. Gently peel off the parchment paper. Starting from one of the short ends, roll the cake up tightly in the towel. Let it cool completely in the towel (about 30 minutes).
Step 5: Prepare the Chocolate Ganache: In a medium saucepan, heat the heavy cream over medium heat until it just begins to simmer. Remove from heat and pour over the chopped chocolate. Let it sit for 1 minute to soften the chocolate. Gently whisk until smooth and glossy. Stir in the softened butter until melted and incorporated. Let the ganache cool slightly until it thickens enough to spread (about 15-20 minutes).
Step 6: Prepare the Filling: In a cold bowl, whip the heavy cream with powdered sugar and vanilla extract until stiff peaks form. Be careful not to overwhip.
Step 7: Assemble the Yule Log: Carefully unroll the cooled cake. Spread the whipped cream filling evenly over the cake. Re-roll the cake tightly without the towel. Trim the ends of the log to create clean edges.
Step 8: Frost and Decorate: Place the yule log on a serving platter. Spread the chocolate ganache evenly over the entire log. Use a fork to create a bark-like texture in the ganache. Optionally, cut a small piece of the log off at an angle and attach it to the side of the log to resemble a branch. Decorate with powdered sugar, meringue mushrooms, sugared cranberries, chocolate shavings, and rosemary sprigs, as desired.
Step 9: Chill and Serve: Refrigerate the yule log for at least 1 hour to allow the ganache to set. Before serving, dust with additional powdered sugar, if desired.
Optional Tip: For a coffee-flavored yule log, add 1 tablespoon of instant espresso powder to the hot heavy cream when making the ganache. For a gluten-free version, substitute a gluten-free all-purpose flour blend for the regular flour.