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1
Preheat the oven to 350.
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2
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.
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3
Butter the paper and dust with flour.
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4
In a small bowl, whisk the 1 cup of flour with the cocoa and salt.
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5
In another small bowl, combine the chocolate and espresso.
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6
In the bowl of a standing electric mixer, combine the egg yolks with 2/3 cup of the sugar.
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7
Set the bowl over a pan of simmering water and whisk until the sugar is dissolved.
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8
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.
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9
Beat in the melted chocolate mixture along with the vanilla.
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10
Transfer to a large bowl.
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11
Thoroughly wash and dry the mixer bowl and the whisk.
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12
Beat the egg whites with the cream of tartar on moderately high speed until soft peaks form.
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13
Gradually add the remaining 2/3 cup of sugar and continue beating at high speed until the whites are glossy, about 2 minutes longer.
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14
Whisk one-fourth of the egg whites into the cake batter, then fold in the remaining whites until no streaks remain.
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15
In a small bowl, whisk the melted butter with 1/2 cup of the batter; fold this mixture into the batter.
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16
Working in 2 batches, sift the cocoa powder mixture over the batter and gently fold it in.
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17
Divide the batter between the prepared pans, spreading it out to fill the pans.
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18
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.
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19
Transfer the pans to racks to cool completely.
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20
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.
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21
Spread the Malted Buttercream over the cakes.
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22
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.
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23
Roll one strip into a tight coil, leaving the paper behind.
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24
Roll the 3 remaining cake strips around the coil in the same way to form a very wide, short jelly roll.
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25
Set the cake on a large plate, spiraled end up.
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26
Frost the outside of the cake with the Dark Chocolate Buttercream.
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27
Refrigerate until set, at least 8 hours.
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28
Decorate the cake with meringue mushrooms, cranberries and rosemary sprigs and serve, cutting the cake into wedges or horizontal slices.
