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1
Preheat the oven to 375.
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2
Butter an 11 1/2-by-17-inch jellyroll pan and line the bottom with parchment paper.
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3
Butter the parchment.
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4
In a food processor, finely grind the hazelnuts.
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5
In a medium bowl, combine the ground hazelnuts with the flour and cinnamon.
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6
In a large bowl, using a handheld electric mixer, beat the egg yolks with 1/2 cup of the sugar at high speed until light yellow, about 3 minutes.
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7
In another large bowl, using clean beaters, beat the egg whites with a pinch of salt with the electric mixer at high speed until soft peaks form.
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8
Gradually add the remaining 1/4 cup of sugar and beat at high speed until the egg whites are firm and glossy.
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9
With a large spatula, fold one-fourth of the dry ingredients into the egg yolk mixture.
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10
Fold in one-third of the beaten egg whites.
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11
Working in 2 batches, fold in the remaining dry ingredients and egg whites.
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12
Fold in the melted butter until just incorporated.
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13
Scrape the batter into the prepared jelly-roll pan and smooth the surface.
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14
Bake for 10 minutes, or until the cake is golden and springs back when lightly touched.
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15
Let cool.
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16
Using the edges of the parchment paper, lift the cake out of the jelly-roll pan.
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17
Trim the edges and cut the cake into four 4 1/2-by-8 1/2-inch rectangles.
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18
Line each of two 9-by-5-inch glass loaf pans with a cake rectangle.
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19
In a small bowl, mix the maple syrup with the Armagnac until blended; brush half of the syrup on the cakes in the loaf pans.
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20
Reserve the remaining syrup and cover the 2 remaining cake rectangles with plastic wrap.
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21
Prepare an ice water bath in a medium bowl.
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22
In a small saucepan, combine the sugar and water and bring to a boil.
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23
Simmer over moderately high heat until the sugar syrup reaches 220 on a candy thermometer, about 8 minutes.
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24
In a heatproof medium bowl, beat the egg yolks with a handheld electric mixer until light yellow, about 3 minutes.
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25
While beating the yolks at high speed, carefully and slowly pour in the hot sugar syrup; the mixture will thicken.
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26
Set the bowl in the ice water bath and beat the egg yolk mixture until chilled and very thick, about 4 minutes.
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27
In large bowl, whip the cream to stiff peaks.
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28
Fold in the Armagnac and vanilla.
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29
Fold in the egg yolk mixture.
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30
Fold in the hazelnuts and chopped chocolate.
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31
Scrape the mousse into the loaf pans.
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32
Brush one side of the remaining 2 cake rectangles with the remaining maple-Armagnac syrup; set them, brushed side down, over the mousse.
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33
Cover with plastic and freeze overnight or for up to 5 days.
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34
To serve, fill a large bowl with 4 inches of very hot water and dip the bottoms of the loaf pans in the water for 10 seconds.
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35
Invert the cakes onto platters.
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36
With a sharp knife, slice the cakes crosswise 1 inch thick and transfer to plates.
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37
Drizzle the slices with the melted chocolate and serve.
