How to Make It
Heat oven to 350° F. Butter a 9-by-13-inch jelly roll pan and line with parchment; butter the parchment. Whisk together the flour, cocoa, and salt in a small bowl.
Beat the egg yolks, vanilla, and ¼ cup of the sugar in a medium bowl with an electric mixer on medium-high until light yellow, 3 to 4 minutes.
In a separate bowl, beat the egg whites (with clean beaters) on medium-high until foamy, 1 to 2 minutes. Gradually add ½ cup of the remaining sugar and beat until stiff peaks form, 2 to 3 minutes more.
Gently fold the egg yolk mixture into the egg whites. Gradually fold in the dry ingredients. Transfer the mixture to the prepared pan and use an offset spatula to smooth the surface. Bake until the cake springs back lightly when pressed, 8 to 10 minutes. Cool.
Whip the heavy cream, sour cream, and the remaining 3 tablespoons of sugar in a medium bowl with an electric mixer on medium-high until stiff peaks form, 1 to 2 minutes.
Spread the whipped cream on the cake, leaving a 2-inch border along one of the short ends. Starting at the other short end, roll up the cake toward the uncovered border (letting the parchment fall away as you roll). Place the cake seam-side down on a serving platter. Drizzle the chocolate ganache over the top; let set for 5 minutes. Run the tines of a fork through the ganache to create “bark.” (“Bûche de Noël” is French for “Yule log,” after all.)