Preheat oven to 375°. Coat an 18×13″ rimmed baking sheet or jelly roll pan with nonstick vegetable oil spray and line with parchment paper. Spray parchment. Place a large kitchen towel (it should be larger than baking sheet) on a work surface. Whisk equal parts powdered sugar and unsweetened cocoa powder in a small bowl to combine. Using a fine-mesh sieve, dust towel with cocoa mixture.
Sift ¼ cup (31 g) all-purpose flour and ¼ cup (21 g) unsweetened cocoa powder through sieve into a small bowl. Place 4 oz. bittersweet chocolate, coarsely chopped, 2 Tbsp. vegetable oil, and ½ tsp. vanilla extract in a small microwave-safe bowl and microwave in 15-second bursts, stirring between each, until melted. Let cool slightly.
Using an electric mixer on medium-high speed, beat 6 large egg whites, room temperature, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a large bowl until foamy. With motor running, gradually add ⅓ cup (67 g) granulated sugar and beat until stiff peaks form and meringue is glossy.
Using clean beaters, beat 3 large egg yolks, room temperature, and ⅓ cup (67 g) granulated sugar in another large bowl until pale and thick, about 4 minutes. Beat melted chocolate mixture into yolk mixture until no streaks remain.
Use a large rubber spatula to fold in meringue, leaving some streaks. Fold in dry ingredients just to combine (do not to overmix).
Scrape batter onto prepared pan; smooth into an even layer. Bake until top of cake is dry and springs back when gently pressed and edges are starting to pull away from sides of baking sheet, 10−12 minutes; let cool slightly.
Run a knife along edges of baking sheet to loosen, then invert cake onto prepared towel. Carefully peel away parchment. Starting at one of the long sides, gently roll up warm cake into a log inside towel. Transfer, seam side down, to a wire rack and let sit until cake is just barely warm, about 20 minutes.
Combine remaining 2 Tbsp. granulated sugar and 2 Tbsp. hot water in a jar, seal, and shake vigorously until sugar is dissolved, about 1 minute. Uncover and stir in 2 Tbsp. cognac, dark rum, or brandy. Unroll cake and brush top with cognac syrup.
Do ahead: Cake can be made 1 day ahead; transfer to baking sheet, cover with plastic wrap, and store at room temperature.
Place 1 Tbsp. cold water in a small bowl and sprinkle ¾ tsp. unflavored powdered gelatin over; set aside.
Meanwhile, bring 6 oz. vacuum-packed or jarred roasted chestnuts, 1 cup whole milk, ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, and ½ tsp. vanilla extract to a simmer in a small saucepan and cook until chestnuts are falling apart and milk is reduced by half, 12−18 minutes. Add gelatin mixture, stirring to dissolve.
Transfer chestnut mixture to a food processor and process until very smooth. Let chestnut cream cool.
Whisk 1 cup heavy cream in a medium bowl until soft peaks form.
Using electric mixer on medium-high speed, beat 2 large egg yolks and ¼ cup (50 g) granulated sugar in another medium bowl until pale and thick, about 4 minutes. Beat in chestnut cream. Using a rubber spatula, gently fold in whipped cream just until incorporated.
Spread chestnut mousse over top of cake, leaving a 1″ border. Roll up cake and place, seam side down, and return to baking sheet. Chill until mousse is set, at least 3 hours.
Preheat oven to 200°. Using electric mixer on medium-high speed, beat 2 large egg whites, room temperature, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a clean medium bowl until foamy. With motor running, gradually add ⅓ cup granulated sugar and beat until stiff, glossy peaks form. Add ⅓ cup powdered sugar and 1 tsp. distilled white vinegar and beat until meringue is very dense (peaks should still be stiff and glossy). Transfer meringue to piping bag fitted with medium round tip.
Pipe 1″-long cones (these will be the mushroom stems) and quarter-size rounds (the mushroom caps) onto a parchment-lined baking sheet. Using a fine-mesh sieve, dust mushroom caps with unsweetened cocoa powder. Bake, rotating once, until meringues are dry, 1½−2 hours.
Using a small paring knife, make a small hole in center of flat side of mushroom caps. Working one at a time, spread a dab of chocolate (from 2 oz. bittersweet or semisweet chocolate, melted, cooled slightly) on flat side of mushroom caps with an offset spatula and fit smaller end of mushroom stems into hole, pressing to adhere. Let sit until chocolate hardens.
Do Ahead: Meringue mushrooms can be made 3 days ahead. Store airtight at room temperature.
Chocolate Buttercream and Assembly
Using electric mixer on medium-high speed, beat 2 large egg yolks, 1 large egg, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt until pale and thick, about 4 minutes.
Meanwhile, fit a small saucepan with candy thermometer and bring ½ cup (100 g) granulated sugar and ¼ cup water to a boil over medium-high heat, stirring with a heatproof rubber spatula to dissolve sugar. Continue to cook, swirling occasionally (but not stirring), until thermometer registers 248° (this is the firm-ball stage).
With the motor running, gradually pour sugar syrup around edges of egg mixture where eggs meet sides of bowl (you want to avoid the beaters to prevent syrup from splattering). Beat until mixture is light and fluffy and outside of bowl is cool to the touch, about 5 minutes.
Gradually beat in 1 cup (2 sticks) unsalted butter, room temperature, cut into pieces, a piece at a time, mixing until incorporated before adding the next piece. If mixture looks soupy or curdled, continue to beat until an emulsion forms and mixture is light and thick (this could take several minutes). Beat in 5 oz. bittersweet or semisweet chocolate, melted, cooled, and 1 tsp. vanilla extract.
Using a long serrated knife, trim ½” from each end of cake to create clean edges; discard (or eat!). Working from one end and cutting at a 45° angle, slice off a 2″ piece of cake. Cut another 2″ piece from same end, this time cutting perpendicular to cake roll to create a squared off end.
Dab 1 Tbsp. buttercream on angled sides of each 2″ piece of cake. Place each piece against roll to create branches, positioning one on top and the other on the side.
Using an offset spatula or butter knife, spread buttercream over outside of roll, leaving cut ends exposed to reveal spiral. Use spatula to create textured lines in buttercream to look like tree bark.
Just before serving, dust cake with powdered sugar and decorate with meringue mushrooms.
Do ahead: Chocolate buttercream can be made 2 days ahead; cover and chill, or freeze up to 1 month. Bring to room temperature and beat again before using. (Thaw frozen buttercream in refrigerator overnight before bringing to room temperature.) Bûche de Noël (without powdered sugar and mushrooms) can be made 1 day ahead; cover and chill.
Editor’s note: This bûche de Noël recipe was first printed in December 2014. Head this way for more of our best Christmas desserts →