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