Traditionally served around Christmas time, this classic French Yule Log or Buche de Noel is a show-stopping centerpiece. Comprised of a deliciously light chocolate sponge cake that is moistened with Kahlua and filled with an intense tiramisu filling, the cake is thickly frosted in a rich dark chocolate ganache and topped with pistachio "moss," a rosemary sprig "forest" and a generous flourish of confectioners' sugar "snow." Despite looking rather complex and tricky, this recipe is incredibly simple and straightforward.
Begin by measuring out all the ingredients required.
Preheat the oven to 450 F / 230 C. Grease and line a 9-by-13 inch / 23-by-33 centimeter swiss roll baking sheet or rimmed baking tray with parchment paper, letting the parchment paper slightly overhand the short sides. Grease the top of the parchment lightly and dust the pan with cocoa powder. Tap out any excess.
Combine the Dry Ingredients
In a medium sized bowl, whisk together the flour, cocoa powder, baking powder, 1/2 teaspoon of espresso powder and salt. Set aside.
Whisk the Eggs and Sugar
In the bowl of a stand mixer fitted with the whisk attachment, or with a hand-held electric whisk, whisk together the eggs and granulated sugar on high speed until thick, fluffy and doubled in volume, about 4 to 6 minutes.
The egg and sugar mixture will be ready when it reaches "ribbon stage," meaning that when the whisk is lifted out it will be thick and leave a trail.
Add the dry ingredients into the egg and sugar mixture. Gently fold to combine until just incorporated.
Add the Butter
Gently stream in the butter and carefully fold to just combine.
Pour the mixture into the prepared pan and gently smooth out the top. Bake for 8 to 12 minutes, or until the cake has pulled away from the sides of the pan and springs back when gently touched.
While the cake bakes, evenly dust a clean kitchen tea towel with cocoa powder. Once the cake is baked, use the over-hanging parchment paper to help invert the cake out onto the tea towel. Remove the parchment paper and lightly sprinkle more cocoa powder on top of the cake.
Then working quickly, firmly and gently, use the tea towel to roll up the warm cake from the long side. Tightly seal, then leave the rolled cake to cool in its tea towel sling until cool.
Make the Ganache
Put the chopped chocolate in a large bowl. Heat 3/4 cup of the heavy cream in a medium saucepan over medium-low heat until it reaches a rolling boil. Pour the hot cream over the chopped chocolate and let it stand for a minute before beginning to stir, concentrating in the center, until smooth then stir in the corn syrup. Chill the ganache bowl for up to 30 minutes, stirring a couple of times until it is thick, smooth and spreadable in consistency.
Make the Filling
In the bowl of a stand mixer fitted with the beater attachment, or with a hand-held electric beater, beat together the mascarpone, confectioners' sugar, 1/2 teaspoon espresso powder and 1 tablespoon Kahlua until thick and fluffy, about 3 minutes. Then in a separate bowl, beat the remaining 1/2 cup of heavy cream on medium speed just until it holds stiff peaks. Fold the whipped cream into the mascarpone mixture to loosen and set the filling aside until needed.
Fill the Roulade
Once the cake is cool, carefully unroll it. Moisten the cake generously with the remaining Kahlua then evenly spread over the mascarpone filling. Gently and tightly re-roll the cake to form the roulade.
Assemble the Yule Log
Cut a 2 to 3 inch / 5 to 7 centimeter piece diagonally off the of the roulade to use as the stump. Gently remove the roulade to a serving platter.
Frost the Yule Log
Use the cooled ganache to attach the stump to the top or side of the roulade then use the rest of the ganache to generously frost the rest of the roulade, being generous with the amount that you use. Use a knife or offset spatula to give the ganache a rough "bark-style" look.
Finish by decorating the Yule Log with the crushed pistachios, rosemary sprigs and a generous dusting of confectioners' sugar.
The Yule Log is best served on the day it is made but can be kept covered and refrigerated for up to two days. Let it sit out for 30 minutes to return to room temperature and soften the ganache before serving.