Time: 1 hour 30 minutes
Time (other): overnight chilling
For the dark chocolate and malted buttercreams:
5 egg whites
1 ½ cups sugar
1 teaspoon vanilla extract
2 cups unsalted butter
4 ounces dark chocolate, melted and cooled
¼ cup malt powder
12 malted milk balls candies, crushed
In the bowl of a standing mixer, combine egg whites and sugar. Set bowl over a pot of simmering water, and whisk until sugar dissolves and egg whites are warm.
Place bowl in mixer; fit with whisk attachment. Add vanilla and beat egg whites at high speed until firm and glossy, about 5 minutes. Whisk in butter a few tablespoons at a time.
Transfer 1 ½ cups of buttercream to a bowl, and whisk in melted chocolate. Cover chocolate buttercream and refrigerate.
Dissolve malt powder in 2 tablespoons hot water. Beat it into remaining buttercream in mixer. Beat in crushed malted milk balls. Cover malt buttercream and refrigerate.
For the chocolate cake roll:
1 cup flour
¼ cup dark unsweetened cocoa powder
¼ teaspoon salt
2 tablespoons instant espresso powder
¼ c hot water
12 ounces dark chocolate, melted and cooled
12 large eggs, at room temperature, separated
1 1/3 cups sugar
4 teaspoons pure vanilla extract
¼ teaspoon cream of tartar
6 tablespoons unsalted butter, melted and cooled
Butter two 17-by-12-inch rimmed baking sheets. Line with parchment paper, leaving a 1-inch overhang on the short sides. Butter paper and dust with flour.
In a small bowl, whisk flour, cocoa powder and salt. Set cocoa powder mixture aside. In another small bowl, dissolve espresso powder in 1/4 cup hot water. Stir dark chocolate into espresso powder. Set melted chocolate mixture aside.
In the bowl of a mixer, whisk egg yolks and 2/3 cup sugar. Set the bowl over a pan of simmering water. Whisk until sugar dissolves. Transfer the bowl to the mixer, fitted with whisk attachment, and beat on high until yolks are pale and thick, about 5 minutes. Beat in melted chocolate mixture along with vanilla. Transfer the cake batter mixture to a large bowl.
In a clean mixer bowl, beat egg whites with cream of tartar on medium-high speed until soft peaks form. Gradually add remaining 2/3 cup sugar and continue beating at high speed until whites are glossy, about 2 minutes longer. Whisk a quarter of the egg white mixture into cake batter, then fold in remaining egg white mixture until no streaks remain.
In a small bowl, whisk melted butter with 1/2 cup of batter; then fold into remaining batter. Sift cocoa powder mixture over batter and fold it in. Divide batter between prepared pans; spread evenly.
Bake cakes at 350 F for 18 minutes, until they feel springy and slightly dry. Transfer pans to wire racks to cool completely. Run the tip of a knife around edges, cover with parchment paper and a baking sheet and turn it out of pan; peel off parchment liner.
Spread malted buttercream over cakes. Cut each cake in half lengthwise, cutting through parchment lining; you should have four 6-by-17-inch strips of cake. Roll one strip into a tight coil, removing the paper as you roll. Roll 3 remaining cake strips around the coil the same way to form a very wide, short jellyroll. Set cake on a large plate, spiraled end up. Frost the outside of the cake with chocolate buttercream. Refrigerate until set, at least 8 hours. Decorate the cake with meringue mushrooms, cranberries and rosemary sprigs and serve, cutting the cake into wedges or horizontal slices.