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1
Preheat the oven to 350.
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2
Lightly coat two 12-by-17-inch jelly-roll pans with vegetable oil spray; line them with parchment or wax paper.
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3
In a standing electric mixer fitted with a whisk, beat the egg whites until soft peaks form.
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4
At high speed, gradually add the granulated sugar and beat until the whites are glossy and firm, about 1 minute.
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5
Transfer the beaten whites to a large bowl; rinse and dry the mixing bowl.
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6
Add the egg yolks, butter, confectioners' sugar, lemon zest, vanilla and salt to the mixing bowl and beat at moderate speed until creamy.
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7
Beat in one-fourth of the beaten whites.
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8
Transfer the batter to a large bowl.
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9
Fold in onethird of the flour, followed by onethird of the remaining whites; repeat with the remaining flour and whites until no streaks of white remain.
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10
Divide the batter between the prepared pans and spread evenly with an offset spatula.
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11
Bake the cakes for about 12 minutes, or until firm, shifting the pans halfway through baking.
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12
In a large heatproof bowl set over a saucepan of simmering water, beat the eggs with the sugar until tripled in volume and an instant-read thermometer inserted in the mixture registers 165, about 6 minutes.
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13
Whisk in the vanilla.
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14
Remove the bowl from the pan and pour out the water.
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15
Add the chocolate and water to the pan and cook over moderate heat, stirring, until the chocolate is melted.
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16
Scrape the chocolate into a medium bowl.
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17
Whisk in one-fourth of the egg mixture, then gradually whisk in the rest.
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18
Let cool.
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19
In a large bowl, using a handheld mixer, beat the butter until fluffy.
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20
Beat in the chocolate mixture.
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21
Refrigerate until firm.
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22
Liberally sift confectioners' sugar over the cakes.
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23
Top each jellyroll pan with a baking sheet; invert to release the cakes.
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24
Remove the parchment and dust the cakes with confectioners' sugar.
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25
Cut each cake crosswise into three 5by12inch rectangles.
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26
Place 1 cake layer on a platter.
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27
Beat the filling until thick and fluffy, about 2 minutes; spread 1/2 cup evenly over the layer.
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28
Repeat to make a total of 5 layers, reserving the last cake rectangle and 1/4 cup of filling.
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29
Thinly frost the side of the cake with the remaining 1/4 cup of filling.
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30
Refrigerate until very firm.
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31
Spray a large sheet of wax paper with vegetable oil spray.
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32
Center the remaining cake layer on the sheet.
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33
In a medium saucepan, cook the sugar and water over moderately high heat, stirring, until an amber caramel forms, 5 minutes.
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34
Carefully pour the caramel over the cake layer and spread it thinly with a small offset spatula.
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35
(About one-fourth of the caramel will end up on the paper.)
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36
Working quickly, cut the cake in half crosswise with a buttered sharp knife, then cut it in half crosswise again to form 4 rectangles.
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37
Cut each rectangle into 3 strips to make 12 equal strips.
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38
Trim any excess caramel from the sides.
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39
Let the caramel harden completely.
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40
Arrange the caramel cake strips side by side on the top of the cake.
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41
Cut into slices between the strips and serve.
