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 big kitchen towel (it must be bigger than baking sheet) on a piece floor. Whisk equal elements powdered sugar and unsweetened cocoa powder in a small bowl to mix. Utilizing a fine-mesh sieve, mud towel with cocoa combination.
Sift ¼ cup (31 g) all-purpose flour and ¼ cup (21 g) unsweetened cocoa powder by means of sieve right 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 every, till melted. Let cool barely.
Utilizing an electrical mixer on medium-high velocity, beat 6 massive egg whites, room temperature, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a big bowl till foamy. With motor working, step by step add ⅓ cup (67 g) granulated sugar and beat till stiff peaks kind and meringue is shiny.
Utilizing clear beaters, beat 3 massive egg yolks, room temperature, and ⅓ cup (67 g) granulated sugar in one other massive bowl till pale and thick, about 4 minutes. Beat melted chocolate combination into yolk combination till no streaks stay.
Use a big rubber spatula to fold in meringue, leaving some streaks. Fold in dry substances simply to mix (do to not overmix).
Scrape batter onto ready pan; easy into a good layer. Bake till prime of cake is dry and comes again when gently pressed and edges are beginning to draw back from sides of baking sheet, 10−12 minutes; let cool barely.
Run a knife alongside edges of baking sheet to loosen, then invert cake onto ready towel. Rigorously peel away parchment. Beginning at one of many lengthy sides, gently roll up heat cake right into a log inside towel. Switch, seam facet down, to a wire rack and let sit till cake is simply barely heat, about 20 minutes.
Mix remaining 2 Tbsp. granulated sugar and a couple of Tbsp. sizzling water in a jar, seal, and shake vigorously till sugar is dissolved, about 1 minute. Uncover and stir in 2 Tbsp. cognac, darkish rum, or brandy. Unroll cake and brush prime with cognac syrup.
Do forward: Cake could be made 1 day forward; switch to baking sheet, cowl with plastic wrap, and retailer at room temperature.
Place 1 Tbsp. chilly water in a small bowl and sprinkle ¾ tsp. unflavored powdered gelatin over; put aside.
In the meantime, carry 6 oz. vacuum-packed or jarred roasted chestnuts, 1 cup complete milk, ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, and ½ tsp. vanilla extract to a simmer in a small saucepan and prepare dinner till chestnuts are falling aside and milk is diminished by half, 12−18 minutes. Add gelatin combination, stirring to dissolve.
Switch chestnut combination to a Meals processor and course of till very easy. Let chestnut cream cool.
Whisk 1 cup heavy cream in a medium bowl till gentle peaks kind.
Utilizing electrical mixer on medium-high velocity, beat 2 massive egg yolks and ¼ cup (50 g) granulated sugar in one other medium bowl till pale and thick, about 4 minutes. Beat in chestnut cream. Utilizing a rubber spatula, gently fold in whipped cream simply till integrated.
Unfold chestnut mousse over prime of cake, leaving a 1″ border. Roll up cake and place, seam facet down, and return to baking sheet. Chill till mousse is about, at the least 3 hours.
Preheat oven to 200°. Utilizing electrical mixer on medium-high velocity, beat 2 massive egg whites, room temperature, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a clear medium bowl till foamy. With motor working, step by step add ⅓ cup granulated sugar and beat till stiff, shiny peaks kind. Add ⅓ cup powdered sugar and 1 tsp. distilled white vinegar and beat till meringue could be very dense (peaks ought to nonetheless be stiff and shiny). Switch meringue to piping bag fitted with medium spherical tip.
Pipe 1″-long cones (these would be the mushroom stems) and quarter-size rounds (the mushroom caps) onto a parchment-lined baking sheet. Utilizing a fine-mesh sieve, mud mushroom caps with unsweetened cocoa powder. Bake, rotating as soon as, till meringues are dry, 1½−2 hours.
Utilizing a small paring knife, make a small gap in heart of flat facet of mushroom caps. Working one by one, unfold a dab of chocolate (from 2 oz. bittersweet or semisweet chocolate, melted, cooled barely) on flat facet of mushroom caps with an offset spatula and match smaller finish of mushroom stems into gap, urgent to stick. Let sit till chocolate hardens.
Do Forward: Meringue mushrooms could be made 3 days forward. Retailer hermetic at room temperature.
Chocolate Buttercream and Meeting
Utilizing electrical mixer on medium-high velocity, beat 2 massive egg yolks, 1 massive egg, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt till pale and thick, about 4 minutes.
In the meantime, match a small saucepan with sweet thermometer and convey ½ cup (100 g) granulated sugar and ¼ cup water to a boil over medium-high warmth, stirring with a heatproof rubber spatula to dissolve sugar. Proceed to prepare dinner, swirling often (however not stirring), till thermometer registers 248° (that is the firm-ball stage).
With the motor working, step by step pour sugar syrup round edges of egg combination the place eggs meet sides of bowl (you wish to keep away from the beaters to stop syrup from splattering). Beat till combination is mild and fluffy and out of doors of bowl is cool to the contact, about 5 minutes.
Progressively beat in 1 cup (2 sticks) unsalted butter, room temperature, minimize into items, a bit at a time, mixing till integrated earlier than including the subsequent piece. If combination appears soupy or curdled, proceed to beat till an emulsion varieties and combination is mild and thick (this might take a number of minutes). Beat in 5 oz. bittersweet or semisweet chocolate, melted, cooled, and 1 tsp. vanilla extract.
Utilizing a protracted serrated knife, trim ½” from every finish of cake to create clear edges; discard (or eat!). Working from one finish and reducing at a forty five° angle, slice off a 2″ piece of cake. Reduce one other 2″ piece from identical finish, this time reducing perpendicular to cake roll to create a squared off finish.
Dab 1 Tbsp. buttercream on angled sides of every 2″ piece of cake. Place every bit towards roll to create branches, positioning one on prime and the opposite on the facet.
Utilizing an offset spatula or butter knife, unfold buttercream over outdoors of roll, leaving minimize ends uncovered to disclose spiral. Use spatula to create textured strains in buttercream to appear to be tree bark.
Simply earlier than serving, mud cake with powdered sugar and adorn with meringue mushrooms.
Do forward: Chocolate buttercream could be made 2 days forward; cowl and chill, or freeze as much as 1 month. Carry to room temperature and beat once more earlier than utilizing. (Thaw frozen buttercream in fridge in a single day earlier than bringing to room temperature.) Bûche de Noël (with out powdered sugar and mushrooms) could be made 1 day forward; cowl and chill.
Editor’s observe: This bûche de Noël recipe was first printed in December 2014. Head this manner for extra of our greatest Christmas desserts →