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1
Preheat the oven to 375.
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2
Butter an 18-by-12-inch large rimmed baking sheet and line the bottom with parchment or wax paper.
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3
Butter the paper and dust with flour, tapping out any excess.
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4
In a small saucepan, combine the chocolate with the dissolved espresso and cook over low heat, stirring constantly, until melted, about 2 minutes.
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5
Let cool slightly.
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6
In a medium bowl, using an electric mixer, beat the egg yolks at high speed until pale, 2 to 3 minutes.
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7
At medium speed, gradually beat in 1/3 cup of the sugar.
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8
Increase the speed to high and beat until very thick and pale, about 5 minutes.
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9
Add the vanilla and the chocolate mixture and beat at low speed until blended.
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10
In a large bowl, using clean beaters, beat the egg whites with the cream of tartar and salt until soft peaks form.
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11
Gradually add the remaining 1/3 cup of sugar and beat at high speed until the whites are stiff and glossy.
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12
Using a large rubber spatula, fold the chocolate mixture and the flour into the whites in 2 batches, until no streaks remain.
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13
Spread the batter evenly in the prepared baking sheet.
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14
Bake for 12 minutes, or until the cake has risen and is slightly springy.
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15
Transfer to a rack and let cool.
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16
Run the tip of a knife around the cake and sift cocoa over the surface.
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17
Cover the cake with wax paper and top with a cutting board.
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18
Invert the cake onto the cutting board.
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19
Lift off the pan and peel off the paper.
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20
Cut the cake in half crosswise, forming 2 rectangles.
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21
Chill until firm, about 30 minutes.
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22
In a small saucepan, combine the chocolate with the butter and water.
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23
Cook over very low heat, stirring constantly, until melted and shiny.
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24
Scrape into a bowl and let cool.
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25
Using an electric mixer, beat the cream with the confectioners' sugar and vanilla until firm.
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26
Using a rubber spatula, fold in the cooled chocolate.
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27
In a small saucepan, combine the shortening with the granulated sugar and water and bring to a simmer.
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28
Off the heat, add the bittersweet and unsweetened chocolate and let stand until melted, 2 minutes.
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29
Whisk in the vanilla.
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30
Let the glaze cool until slightly thickened, about 30 minutes.
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31
Spread the cream filling over one half of the cake and top with the other half, cocoa side up.
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32
Press down lightly and run a cake spatula all around to smooth the edges.
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33
Pour the glaze down the center of the cake and gently spread it so that it covers the top and drizzles down the sides.
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34
Freeze the cake for 4 hours or overnight.
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35
Let stand at room temperature until just soft enough to cut.
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36
Cut the cake into 3 long strips, then cut each strip into 6 squares.
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37
Transfer to plates and serve.
