Preheat your oven to 350°F. Line the bottom and sides of a (10x15x1 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 or a handheld whisk, whip the egg whites until they're fluffy and opaque and hold a soft peak. To test for soft peaks, hold your whisk upright (pictured above). The whipped egg whites should curl down slightly. Set aside.
In a large bowl, combine your egg yolks and sugar. Whip together until the mixture almost doubles in volume and turns pale yellow. You should be able to draw a figure 8 in the mixture and it should take a few seconds to disappear (visual pictured above). Add the vanilla and whisk it into the egg yolk mixture.
Add the dry ingredients to the egg yolks. Mix together until you no longer see any dry ingredients. The batter will be very thick at this point.
Fold half of the whipped egg whites into the thick batter to lighten the mixture. Once incorporated, gently fold in the remaining half of the whipped egg whites until just combined! Take care not to deflate the mixture too, too much by over mixing.
Pour the batter into your prepared jelly roll pan. Carefully 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 10 minutes, or until it springs back when lightly touched. 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 cake out onto the parchment paper or kitchen towel. Remove the parchment paper used for baking from the bottom. Dust the top with more powdered sugar or cocoa powder because this cake loves to stick! While the cake is still hot, 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.
Make the chocolate ganache: Place your chocolate in a heat-safe bowl, and set aside. Heat the cream in the microwave or in a small pot on your stovetop until it just starts to bubble along the edges. Carefully pour the cream over the chocolate and let sit for 5 minutes. Whisk the chocolate and cream together until the chocolate is dissolved and everything is smooth and shiny.
Pour the warm ganache over your cooled 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 and rosemary *** (see notes), meringue mushrooms, powdered sugar, or whatever else you like!