Preheat oven to 300ᵒF/149ᵒC. Using an 8-inch plate or pan as a guide, trace the perimeter of the plate or pan onto a sheet of parchment paper. Transfer parchment paper, pencil side down, to a baking sheet. Set aside.
In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites and sugars meringue is stiff and glossy. Sprinkle in cornstarch whisk again to combine.
Mound the meringue in the center of the 8-inch circle. Using a spatula, evenly spread meringue out towards the edges. Transfer baking sheet to oven and immediately reduce the temperature to 250ᵒF/121ᵒC. Bake for 1 hour and 15 minutes.
Turn off oven and let meringue cool completely in oven. Yield: 5 to 7 pieces 6-inched meringue thin wafers
For Chocolate Mousse:
In a sauce pot, whisk egg yolk with sugar and gelatin, then slowly add the cocoa. Heat mixture over medium heat for 2 minutes with continuous stirring.
Remove pot from the heat and continuously stir the chocolate mixture until cooled down.
Add the butter and mix until smooth. Set aside.
Whip the cream to soft peak and fold in the chocolate mixture, until well blended.
In a flat surfaced plate, assemble meringue thin wafer first, then spread the chocolate mousse on top, and sprinkle a portion of chopped hazelnuts chocolate coated wafer and drizzle with melted chocolate.
Repeat procedure up to 5 layers with chocolate mousse on top and lastly sprinkle the rest of the chopped hazelnuts chocolate coated wafers and drizzle with melted chocolate.
Put meringue cake in the freezer for at least an hour to set. Serve.