Preheat your oven to 350°F. Line the bottom and sides of a (12x17 inch) jelly roll pan with parchment paper, and set aside.

In a medium bowl, combine flour, cocoa powder, baking powder, cornstarch, and salt. Whisk to combine, and set aside.

Separate egg whites into a large mixing bowl. Set aside egg yolks for later. Whip the egg whites until fluffy and opaque and hold a stiff peak. Set aside.

In a large bowl, combine egg yolks and sugar. Whip until mixture turns pale yellow.

Add sour cream, oil, and vanilla to the egg yolk mixture. Whisk to combine.

Add dry ingredients to the egg yolk mixture. Whisk until no dry streaks remain.

Fold about ⅓ of the whipped egg whites into the batter. Once incorporated, gently fold in the remaining egg whites until just combined.

Pour the batter into the prepared jelly roll pan. Even out the batter and bake for 15 to 20 minutes, or until it springs back when lightly pressed.

While the cake bakes, lay out a lint-free kitchen towel or piece of parchment paper larger than the cake. Dust with powdered sugar or cocoa powder.

Once baked, turn the hot cake out onto the parchment or kitchen towel. Remove the parchment paper and dust the top with more powdered sugar or cocoa powder. Roll up the cake with the towel/parchment starting from the shorter end. Set seam-side down on a wire rack to cool completely.

In a large bowl, whip heavy cream, powdered sugar, and vanilla until stiff peaks form.

Carefully unroll the cooled cake and spread the whipped cream over it, leaving a small border. Roll the cake back up into a log without the towel/parchment. Chill for at least 1 hour to let the filling set.

For the chocolate ganache, combine chopped chocolate and heavy cream in a heat-safe bowl. Heat until melted and smooth.

Cool the ganache for 5 to 10 minutes, then pour over the yule log cake, spreading evenly along the top and sides. Optionally, create ridges with a fork for decoration.