Reprinted with permission from French Pastry Made Simple by Molly Wilkinson, Page Street Publishing Co. 2021. Photo credit: Joann Pai
See note with American Buttercream instructions if you would like to make this as the filling instead of the French Buttercream. Storage: Chill finished cake until firm and then wrap well in plastic wrap. This will keep for 3 days. Bring to room temperature before serving.
Spray a 10 x 15–inch (25x 38–cm) jelly-roll pan with baking spray. Line with parchment paper (it should go up the long sides, but if it doesn't entirely cover the shorter ends, that’s totally fine; just crease the paper into the corners). Spray the top of the parchment with baking spray as well.
Preheat your oven to 350°F (175°C).
Measure out the flour and set aside with a sieve or sifter nearby. Melt the butter and set aside to cool. In the bowl of a stand mixer fitted with the whisk attachment or in a large bowl, using a hand mixer, put the eggs, egg yolk (for a rolled cake) and sugar. If your eggs feel the least bit cool to the touch, place the whole uncracked egg in a bowl of warm water for several minutes before cracking.
Whip the egg mixture on medium-high speed until tripled in volume, very pale yellow in color (practically white) and mousselike and thick in texture. Lift the beater to check the texture. The batter should fall in a slow, thick ribbon and take a couple of seconds to disappear.
It is important to whip until this texture is achieved. It can take 8 to 10 minutes to get to this point, or sometimes even longer! Set a timer for 8 minutes and check. If it looks as if the eggs aren’t whipping up after several minutes, your mixer speed is too low; in that case, increase the speed slightly. However, it’s important to not whip fully on high speed, or else very big bubbles will be created. Instead, stick to medium-high speed, whipping for a longer time so a sturdier structure is created— this is what creates the lift in the sponge.
Sift the flour over the whipped eggs in two additions, gently folding after each until mostly incorporated. The trick is to keep as much air in the batter as possible. To fold, I repeat in my head, “Into the middle, out to the sides.” Fold with a spatula dipping into the middle, scraping the bottom and then coming up and over the top. After each fold, turn the bowl a little. I’ll scrape around the sides every once in a while, too.
Once the flour is just about incorporated, grab a spatula full of batter and add it to the cooled melted butter. Whisk well to mix the two together. This makes it much easier to blend into the batter. Pour the butter into the batter and fold carefully
Pour the batter in a line down the prepared jelly-roll pan. Use an offset metal spatula to first spread the batter into the corners, and then fill in the rest of the pan. Do this quickly and aim for an even layer.
Immediately place in the preheated oven to keep the batter from deflating. Bake for 9 to 11 minutes. The cake is done when the top is lightly browned across the whole surface, and is pulling away from the sides of the pan.
As soon as you remove it from the oven, cover with a damp, clean tea towel. It’s okay if the cloth touches the cake. This will keep the moisture in and help prevent any cracking. Allow to cool completely in the pan.
Heat the cream just to a simmer and then pour over the chopped chocolate. Wait 1-2minutes and then whisk together to form a ganache. Keep at room temperature to cool and firm up to easily spread on the outside of the Buche. *If needed chill in the refrigerator to speed this up, but keep a close eye on it so it does not get too firm to spread.
Beat the egg yolks several minutes on medium speed with the whisk attachment until slightly lighter in color. Heat the sugar and water to 244F/118C and immediately take off the heat. Let the bubbles calm down for a couple seconds while giving the yolks one last quick whisk. Then slowly pour the hot sugar syrup into the yolks while mixing on low speed. Avoid hitting the whisk with the syrup. Instead, aiming for the gap in between the side of the bowl and the whisk.
Once all of the syrup has been added, turn up the speed on the mixer to medium-high and whip until the bowl is cool to the touch (about 10 minutes). The egg yolks will initially look watery with the syrup addition, then lighten in color and thicken as the two are beaten together.
Meanwhile, melt the milk chocolate and set aside to cool as the buttercream finishes whipping.
Slowly add the very soft butter to the whipped egg yolks, one cube at a time, while whisking on medium speed. Then whip in the melted milk chocolate. If very soft, place in the refrigerator for 5-10 minutes to firm up slightly.
Remove the towel from the cooled genoise cake and transfer to a clean work space, leaving the parchment paper on the bottom. Make sure your serving platter is nearby.
Spread the milk chocolate buttercream on top.
Roll from one of the short ends, using the parchment paper to help. It’s a little awkward at first but be patient, rolling a bit and then peeling away the parchment. Then once you have the roll going, use the parchment to help move the cake. This allows for more even pressure when rolling.
When you’re just about at the end of the roll, transfer the cake and trailing parchment to a serving platter. Roll more to where the end of the cake is on the platter, and pull the parchment is off.
Chill the cake for 15-30 minutes to firm up the buttercream. Trim the ends if needed with a serrated knife.
Spread the ganache on the outside with an offset metal spatula making long indentations to make it look like bark. Cover the ends as well, making a swirl with the end of a kabob stick to mimic the cut off end of the log.
Dust with powdered sugar to top with a little snow and enjoy!
*To make a classic American buttercream as the filling: Whip together 12 tablespoons (170 g) of soft unsalted butter with 2 cups (240 g) of powdered sugar. Then mix in 2oz (60 g) of melted milk chocolate.