In a large bowl, beat egg whites until foamy. Add cream of tartar; beat until soft peaks form. Gradually add sugar, 1 tablespoon at a time, beating until sugar is dissolved and stiff peaks form. Combine hazelnuts and flour; fold into batter, 1/4 cup at a time. Spoon into two greased 9-in. round baking pans lined with waxed paper.
Bake at 300° for 25-30 minutes or until cake springs back when lightly touched. Cool for 10 minutes before removing from pans to wire racks.
In a small heavy saucepan, combine the ganache ingredients; cook and stir over low heat until chocolate and butter are melted. Remove from the heat. Set saucepan in ice; stir for 3-4 minutes or until thickened. Remove from ice and set aside.
For buttercream, combine sugar and water in a large heavy saucepan. Bring to a boil; cook over medium-high heat until sugar is dissolved. Remove from the heat. Add a small amount of hot mixture to egg yolks; return all to pan. Cook and stir 2 minutes longer. Remove from the heat; stir in vanilla. Cool to room temperature.
In a large bowl, cream butter until fluffy. Gradually beat in cooked sugar mixture. Beat in confectioners' sugar until smooth. If necessary, refrigerate until buttercream reaches spreading consistency.
Place one cake layer on a serving plate. Spread with half of ganache to within 1/4 in. of edges. Top with second cake layer and remaining ganache. Freeze for 5 minutes. Spread hazelnuts into sides of cake. Garnish with whole hazelnuts and chocolate leaves. Refrigerate.