Preheat your oven to 350°F. Line the bottom and sides of a (12x17 inch) jelly roll pan with parchment paper, and set aside.
Make the chocolate cake: In a medium bowl, combine your flour, cocoa powder, baking powder, cornstarch, and salt. Whisk to combine, and set aside.
Separate your egg whites into a large mixing bowl * (see notes). Set aside the egg yolks for later. Using an electric mixer fitted with the whisk attachment, whip the egg whites until they're fluffy and opaque and hold a stiff peak. To test for stiff peaks, hold your whisk upright (pictured above). The whipped egg whites should stand up straight. Set aside.
In a large bowl, combine your egg yolks and sugar. Whip together until the mixture turns a pale yellow color. You should be able to quickly draw a figure 8 in the mixture without it disappearing right away (pictured above).
Add the sour cream, oil, and vanilla to the egg yolk mixture. Whisk to combine.
Add the dry ingredients to the egg yolk mixture. Whisk together until you no longer see any streaks of dry ingredients. The batter will be thick at this point.
Fold about 1/3 of the whipped egg whites into the thick batter to lighten up the mixture. Once incorporated, gently fold in the remaining whipped egg whites until just combined! The mixture will deflate slightly, but take care not to over mix or the batter will lose too much airiness.
Pour the batter into your prepared jelly roll pan. Even out the batter (again being careful not to deflate the mixture too much). Bake the cake in the center of your preheated 350°F oven for 15 to 20 minutes, or until it springs back when lightly pressed. A toothpick inserted into the center should also come out clean. ** (see notes)
While the cake bakes, lay out a lint free kitchen towel or piece of parchment paper that's slightly larger than the cake. Dust with powdered sugar or cocoa powder.
Once baked, take the cake out of the oven, and turn the hot cake out onto the parchment paper or kitchen towel. Remove the parchment paper off the bottom of the cake. Dust the top with more powdered sugar or cocoa powder (this cake loves to stick!) Carefully roll up the cake along with the towel/parchment starting from the shorter end. Set aside (seam side down) on a wire rack to cool completely to room temperature.
Make the whipped cream: In a large bowl, whip together your heavy cream, powdered sugar, and vanilla until it reaches stiff peaks.
Carefully unroll your cooled cake. Spread your whipped cream onto the cake in an even layer, leaving a small border along the sides (about 1/2 an inch). Roll your cake back up into a log (this time without the parchment paper/towel). Chill the cake for at least 1 hour to let the filling set.
(Optional) Once your cake is cooled, slice a piece off the tip on a slight angle, and place it on the side of the cake to create a branch (pictured above). Set aside.
Make the chocolate ganache: In a heat safe bowl, combine your chocolate and heavy cream. Heat in a microwave or over a bain-marie until melted.If melting in a microwave, heat your chocolate and cream on high in 10 second bursts, pausing after each burst to mix, until fully melted and smooth.If melting over a bain-marie, place your bowl over a pot of barely simmering water, making sure the bottom of the bowl doesn't touch the water. Heat, stirring occasionally until melted and smooth. Cool the ganache for 5 to 10 minutes, and then pour the warm ganache over your yule log cake. Spread evenly along the top and sides of the cake.(Optional) Let sit for 5 to 10 minutes, then go in and create ridges using a fork to resemble wood grain! Decorate with sugared cranberries & rosemary, meringue mushrooms, powdered sugar, or whatever else you like!