Easy Chocolate Yule Log Cake

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1. 1Step 1: Preheat oven to 350°F. Line a 17x12 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. 2Step 2: Whisk the flour, cocoa, baking powder and salt together in a medium bowl and set aside.
3. 3Step 3: In a large bowl, combine the egg yolks and sugar and whisk together until well combined.
4. 4Step 4: Add the sour cream, melted butter and vanilla extract and whisk together until well combined.
5. 5Step 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. 6Step 6: Add the egg whites to a large mixer bowl and whip on high speed until stiff peaks form.
7. 7Step 7: Gently fold about 1/3 of the whipped egg whites into the chocolate mixture to loosen up the batter.
8. 8Step 8: Add the remaining egg whites and gently fold together until well combined.
9. 9Step 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 touched and a toothpick inserted comes out clean.
10. 10Step 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. 11Step 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. 12Step 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. 13Step 13: Add the mascarpone cheese to the whipped cream and whip until stiff peaks form. It will happen fairly quickly.
14. 14Step 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. 15Step 15: Spread the filling evenly onto the unrolled cake, then roll it back up without the parchment paper.
16. 16Step 16: Wrap it up in plastic wrap with the seam side down and refrigerate for at least an hour to firm up.
17. 17Step 17: When you're ready 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. 18Step 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. 19Step 19: Whip on high speed until lightened in color and thick enough to spread.
20. 20Step 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. 21Step 21: Use some of the chocolate ganache to attach the small log to the side of the larger log.
22. 22Step 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. 23Step 23: Refrigerate the cake until ready to serve.