meringue: Combine egg whites and sugar in the top of a double boiler. Heat to lukewarm. Remove from heat. Pour into mixing bowl and beat about 15 minutes at medium speed, until stiff. Quickly fold in the cocoa to prevent the meringue from falling. Grease a 1 inch springform pan, line with parchment paper, and grease again. Pipe meringue through a pastry bag using the round tube, forming a spiral on the bottom of the springform pan, starting from the center and working out toward the sides until the bottom is completely covered. Bake at 25 degrees for 2 hours. Allow to cool in the pan.
Genoise: Grease a 1x3 inch round baking pan, line with baking parchment, and grease again. Combine beaten eggs and sugar in the top of the double boiler. Heat to lukewarm. Remove from the heat. Pour into a mixing bowl and beat on high speed about 1 minutes, or until the batter forms a ribbon. Sift the flour and cocoa into the batter and fold to combine. Fold in the butter. Pour into the prepared pan and bake at 35 degrees 35 to 4 minutes, or until the cake springs back when lightly touched and a toothpick comes out clean. Cool thoroughly. Remove from pan.
mousse: Melt chocolate in the top of a double boiler over simmering water. Remove pan from the heat and cool. Beat in the egg yolks, one at a time. Beat the egg whites until soft peaks form. Lightly fold a quarter of the beaten egg whites into the chocolate. Fold in the remaining egg whites, lightly but thoroughly. Chill.
Ganache: Heat chocolate and cream together in the top of the double boiler over simmering water until the chocolate melts. Blend well.
Assembly: Spoon half the ganache over the meringue layer in the pan and smooth evenly over the surface. Cut the chocolate Genoise into two 1/4 inch slices, reserving the rest of the cake for another use. Place one slice over the ganache and meringue layer. Cover with about half the mousse. Top with the second layer of cake. Spoon the remaining mousse over the cake and chill until set, at least four to six hours.
Just before serving, spoon remaining ganache on the cake. Carefully line the sides of the cake with chopped toasted almonds. Decorate the top with hazelnut roses by placing the whole hazelnuts evenly around the top of the cake, then place almond slices around each hazelnut to resemble petals