Bûche de Noël with Mascarpone Cream and Dark Chocolate Ganache

Preheat oven to 350°F. Lightly coat an 18-by-13-inch half sheet pan with cooking spray, and line with parchment paper. Lightly coat parchment with cooking spray. Set aside. Place egg yolks, vanilla, salt, 1/2 cup of the cocoa, and 1/4 cup of the granulated sugar in a medium bowl. Beat with an electric mixer on low speed, gradually increasing speed to medium-high until mixture is thick and light brown, 2 to 3 minutes total. Set aside.

Beat egg whites in bowl of a stand mixer fitted with whisk attachment (or in a clean mixing bowl using a hand mixer) on medium speed until foamy, about 30 seconds. Add cream of tartar, and beat on medium speed until soft peaks form, 1 to 2 minutes. With mixer on medium speed, gradually add remaining 3/4 cup sugar, beating until stiff peaks form, about 3 more minutes. Stir one-third of the egg whites into yolk mixture to lighten. Gently fold in remaining egg whites until just combined. Spread batter in prepared pan, and smooth top.

Bake until cake springs back when touched, 14 to 16 minutes. Cool in pan on a wire rack 5 minutes. Working quickly and using a fine-mesh strainer, dust 2 tablespoons of the cocoa evenly over cake. Run a knife along sides of cake; invert onto a parchment paper-lined cutting board. Carefully peel off and discard top layer of parchment from cake. Dust remaining 2 tablespoons cocoa evenly over cake. Starting at one short side, carefully and tightly roll up cake along with bottom layer of parchment together. Transfer cake roll to a wire rack, seam side down, and cool completely, about 45 minutes (you’ll remove the parchment later).

Combine all ingredients in bowl of a stand mixer fitted with whisk attachment (or in a large mixing bowl). Beat on low speed with the electric mixer, gradually increasing speed to medium-high, until stiff peaks form, about 1 minute (do not over-beat or filling will turn grainy).

Carefully unroll cake. Evenly spread filling over cake, leaving a 1/2-inch border. Starting at 1 short side, reroll cake, peeling parchment back as you go. Place cake, seam side down, on a serving platter. Cover loosely with plastic wrap, and chill at least 2 hours or up to overnight.

Place chocolate, corn syrup, and salt in a medium bowl; set aside. Heat heavy cream and butter in a small saucepan over medium, stirring often, until mixture is hot and butter is melted, 3 to 4 minutes. Pour cream mixture over chocolate mixture; let stand, undisturbed, for 2 minutes. Stir until mixture is smooth and chocolate is melted. Cool at room temperature until thick and spreadable, about 2 hours. Do not chill.

Using a serrated knife, trim about 1/4 inch from each end of the cake roll, exposing the filling. Cut a 2 1/2-inch piece of cake from one end. With cake piece still lying on its side, cut it in half diagonally, creating two wedges. Stir Dark Chocolate Ganache Frosting until smooth. Spread about 1 tablespoon frosting onto diagonal cut side of each cake wedhe, and arrange each wedge in a different position on top of the cake roll, pressing to adhere and create branch “stumps.” Using a small offset spatula, spread about 2 tablespoons frosting around base of each stump to adhere to log. Chill cake, uncovered, until frosting has hardened, about 20 minutes. If desired, stir almonds into remaining frosting. Using an offset spatula, spread remaining frosting over log and stumps, leaving ends exposed. Serve Bûche de Noël immediately, or chill, loosely covered with plastic wrap, up to 12 hours. If chilled, let cake stand at room temperature for 30 minutes before serving.

To serve, dust cake lightly with powdered sugar to resemble snow. Garnish log and individual servings with Meringue Mushrooms.