How to Make It
Preheat the oven to 350°. Butter two 12-by-17-inch rimmed baking sheets and line them with parchment paper, leaving a 1-inch overhang on all of the short sides. Butter the paper and dust with flour.
In a small bowl, whisk the 1 cup of flour with the cocoa and salt. In another small bowl, combine the chocolate and espresso. In the bowl of a standing electric mixer, combine the egg yolks with 2/3 cup of the sugar. Set the bowl over a pan of simmering water and whisk until the sugar is dissolved. Transfer the bowl to the mixer fitted with the whisk and beat at high speed until the yolks are pale and thick, about 5 minutes. Beat in the melted chocolate mixture along with the vanilla. Transfer to a large bowl.
Thoroughly wash and dry the mixer bowl and the whisk. Beat the egg whites with the cream of tartar on moderately high speed until soft peaks form. Gradually add the remaining 2/3 cup of sugar and continue beating at high speed until the whites are glossy, about 2 minutes longer. Whisk one-fourth of the egg whites into the cake batter, then fold in the remaining whites until no streaks remain.
In a small bowl, whisk the melted butter with 1/2 cup of the batter; fold this mixture into the batter. Working in 2 batches, sift the cocoa powder mixture over the batter and gently fold it in. Divide the batter between the prepared pans, spreading it out to fill the pans. Bake for about 18 minutes, until the cake feels springy and dry; shift the pans from top to bottom and front to back halfway through baking. Transfer the pans to racks to cool completely. Run the tip of a knife around the edges, cover with parchment paper and a baking sheet and invert; peel off the parchment on top.
Spread the Malted Buttercream over the cakes. Using a ruler, cut each cake precisely in half lengthwise, cutting through the paper; you should have four 6-by-17-inch strips of cake. Roll one strip into a tight coil, leaving the paper behind. Roll the 3 remaining cake strips around the coil in the same way to form a very wide, short jelly roll. Set the cake on a large plate, spiraled end up. Frost the outside of the cake with the Dark 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.
Meringue mushrooms are available at 1-800-bakery.com.