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1
Preheat oven to 300F.
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2
Line large baking sheet with parchment paper.
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3
Draw three 9 x 5-inch rectangles on parchment.
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4
Turn paper over.
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5
Finely grind 1 cup nuts with 1/4 cup sugar in processor.
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6
Coarsely chop 1/4 cup nuts; set aside.
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7
Beat whites and cream of tartar in large bowl until foamy.
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8
Gradually add 1/2 cup sugar, beating until stiff and glossy.
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9
Fold ground hazelnut mixture into whites in 2 additions.
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10
Divide meringue among traced rectangles on parchment, spreading to edges.
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11
Bake meringues until crisp and light golden, about 1 hour 30 minutes.
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12
Turn oven off; cool meringues in oven 1 hour.
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13
Remove from oven.
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14
Bring 1 1/2 cups cream and 3 tablespoons corn syrup to simmer in medium saucepan.
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15
Remove from heat.
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16
Add semisweet chocolate; stir until smooth.
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17
Chill until slightly thickened but still spreadable, about 1 hour.
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18
Line 9 x 5 x 2 1/2-inch metal loaf pan with plastic wrap, leaving 3-inch overhang on all sides.
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19
Run thin knife under meringues to loosen from parchment.
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20
Gently transfer meringues to work surface.
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21
Using serrated knife and gentle sawing motion, trim each to 8 x 4-inch rectangle.
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22
Spread 1/3 cup chocolate ganache over each of 2 meringues.
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23
Place on baking sheet.
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24
Freeze until ganache is set, 10 minutes.
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25
Place third meringue, smooth side down, in prepared loaf pan.
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26
Spread half of White Chocolate Mousse over.
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27
Sprinkle with reserved 1/4 cup chopped hazelnuts.
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28
Top with 1 meringue, ganache side down.
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29
Spread remaining mousse over.
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30
Top with remaining meringue, ganache side down.
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31
Fold plastic overhang over cake; freeze overnight.
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32
Chill remaining semisweet chocolate ganache.
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33
Bring remaining 1/2 cup cream and 1 tablespoon syrup to simmer in small saucepan.
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34
Remove from heat.
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35
Add white chocolate; stir until smooth.
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36
Cover and chill until thick enough to spread.
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37
Invert cake onto small baking sheet; remove plastic.
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38
Working quickly, spread white chocolate ganache thinly over top and sides of cake.
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39
Place 24 hazelnuts around top edge of cake.
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40
Freeze cake until frosting is firm, about 30 minutes.
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41
(Can be made 1 week ahead.
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42
Cover cake tightly and keep frozen.
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43
Keep semisweet chocolate ganache chilled.
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44
Before serving, rewarm ganache over low heat, stirring just until liquid.)
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45
Serve cake, passing remaining semisweet ganache as sauce.
