Make the Chocolate Sponge Cake:

1. Prepare Your Baking Sheet: Preheat your oven to 350°F (175°C). Butter the bottom and sides of an 18×13 inch rimmed baking sheet. Line the bottom with parchment paper and lightly flour the sides, tapping out any excess flour to prevent sticking.
2. Mix Wet Ingredients: In a medium bowl, whisk together the sour cream, vegetable oil, and vanilla extract until well combined. Set this mixture aside.
3. Combine Dry Ingredients: In another bowl, whisk together the all-purpose flour, cocoa powder, baking powder, baking soda, cornstarch, and salt until thoroughly mixed. This step is crucial for even distribution of the leavening agents and cocoa.
4. Whip Eggs and Sugar: In a stand mixer bowl fitted with a whisk attachment, whip the eggs and granulated sugar on high speed for 6 to 8 minutes until the mixture is fluffy, pale yellow, and has tripled in volume. This aeration is key to achieving a light sponge cake.
5. Fold Ingredients Together: Sift the dry ingredients into the egg mixture and add the sour cream mixture. Using a spatula, gently fold the ingredients together until just combined, being careful not to overmix, which can deflate the batter.
6. Spread and Bake: Pour the batter into the prepared baking sheet, spreading it into an even layer. Bake for 12 to 18 minutes, or until the edges start to pull away from the pan and a skewer inserted into the center comes out clean.
7. Cool the Cake: Once baked, dust the cake with cocoa powder, lay a piece of parchment paper over the top, and carefully flip it onto a wire rack to cool. Remove the original parchment paper and allow the cake to cool completely.

Make the Ganache:

1. Melt Ingredients: In a medium saucepan, gently warm the heavy cream and finely chopped chocolate over low heat, whisking continuously until smooth and fully melted.
2. Cool and Thicken: Transfer the ganache to a bowl and let it cool to room temperature until it reaches a spreadable consistency. If it remains too soft, refrigerate it for 15 to 20 minutes, monitoring closely to prevent it from becoming too firm.

Make the Whipped Cream Filling:

1. Combine Ingredients: In a large bowl, combine the cold heavy cream, powdered sugar, cocoa powder, and vanilla extract.
2. Whip to Stiff Peaks: Using an electric mixer on high speed, whip the mixture until stiff peaks form. Be cautious not to over-whip, as this can lead to a grainy texture.

Assemble and Decorate:

1. Spread Filling: Evenly spread the whipped cream filling over the cooled cake, reaching all corners to ensure every bite is delicious.
2. Cut and Roll: With a sharp knife, cut the cake into 4 strips parallel to the short end. Roll one strip into a log, and then continue adding the other strips one by one to form a larger log.
3. Chill Cake: Place the assembled cake swirl side up on a serving plate and chill in the refrigerator for at least 30 minutes. This will help the structure hold while you apply the ganache.
4. Apply Ganache: Once chilled, spread the ganache over the entire cake, using a fork to create wood grain ridges and a spoon to add swirls on top, mimicking the appearance of a real log.
5. Decorate and Serve: Optionally, adorn your Vertical Bûche de Noël with sugared cranberries and rosemary for a festive touch. Slice gently and enjoy the beautiful spiral pattern within!