Yule Log Cake (Bûche de Noël)

1. Preheat oven to 350°F. Line a 17×12 inch jelly roll sheet pan with parchment paper. Make sure the parchment paper sticks up at least an inch above the sides of the pan on all sides. You’ll use the parchment paper later to lift the cake out of the pan and roll it up.
2. Whisk the flour, cocoa, baking powder and salt together in a medium bowl and set aside.
3. In a large bowl, combine the egg yolks and sugar and whisk together until well combined.
4. Add the sour cream, melted butter and vanilla extract and whisk together until well combined.
5. Add the dry ingredient mixture (previously set aside) to the wet ingredient mixture and gently whisk together until well combined, then set aside.
6. Add the egg whites to a large mixer bowl and whip on high speed until stiff peaks form.
7. Gently fold about 1/3 of the whipped egg whites into the chocolate mixture to loosen up the batter.
8. Add the remaining egg whites and gently fold together until well combined.
9. Spread the cake batter evenly into the prepared pan and bake for 10-12 minutes, or until the top of the cake springs back when toughed and a toothpick inserted comes out clean.
10. Remove the cake from the oven and immediately lift the cake out of the pan using the parchment paper and place it on the counter. 
11. While the cake is hot, use the parchment paper the cake was baked in and start at the shorter end of the cake to slowly roll the cake up. You can just set the cake roll aside to cool completely, if you like. But to help avoid having the cake crack later, I like to unroll and re-roll the cake a few times during cooling to help it keep its elasticity. I first unroll it after about 5 minutes, then after another 5 minutes, and again after another 10 minutes. From there, I might roll it out 2-3 more times after 10-15 minute intervals. It should take about an hour to an hour and a half to cool completely, depending on the temperature of your home. You’ll want to fill it once it’s completely cool. Don’t let it sit for several hours.
12. When the cake has cooled and is ready to be filled, make the filling. Add the heavy whipping cream, powdered sugar, vanilla extract and salt to a large mixer bowl and whip on high speed until soft peaks form.
13. Add the mascarpone cheese to the whipped cream and whip until stiff peaks form. It will happen fairly quickly.
14. Unroll the cake roll very carefully, looking out for areas where it may be sticking to release it. You can use an offset spatula or something similar and run it along the parchment paper as you unroll the cake to help release it as it unrolls.
15. Spread the filling evenly onto the unrolled cake, then roll it back up without the parchment paper.
16. Wrap it up in plastic wrap with the seam side down and refrigerate for at least an hour to firm up.
17. When you’re read to decorate the cake, make the chocolate ganache. Add the chocolate to a medium sized bowl and set aside. Heat the cream in the microwave just until it begins to boil, then pour it over the chocolate.
18. Allow the chocolate and cream to sit for a few minutes, then whisk until smooth. Let the ganache cool to about room temperature (or cooler, you don’t want it too warm/thin), then transfer to a large mixer bowl.
19. Whip on high speed until lightened in color and thick enough to spread.
20. To decorate the cake, use a large serrated knife to gently cut off a piece of the log about 3 inches in length. Make the cut with a slight diagonal.
21. Use some of the chocolate ganache to attache the small log to the side of the larger log.
22. Spread the remaining chocolate ganache all over the cake, then use a fork to create bark-like lines all over it. Decorate with sugared cranberries and rosemary (instructions in notes), if desired.
23. Refrigerate the cake until ready to serve.