Prepare the Meringue:

1. Preheat the Oven: Preheat your oven to 275°F (135°C). Line two 9-inch cake pans with parchment paper.
2. Whip the Egg Whites: In a clean, dry bowl, beat the egg whites with an electric mixer on medium speed until they form soft peaks. Add the cream of tartar, and then gradually add the sugar, one tablespoon at a time, while continuing to beat until stiff peaks form.
3. Add Vanilla: Stir in the vanilla extract.
4. Fold in Hazelnuts: Gently fold in the toasted, chopped hazelnuts using a spatula to avoid deflating the meringue.
5. Shape the Meringue: Divide the meringue mixture evenly between the prepared pans, smoothing it into an even layer. Use a spoon or spatula to create a slight dome in the center of each layer to allow for even baking.
6. Bake: Bake the meringue layers for 1 hour to 1 hour 30 minutes, or until the meringues are dry and crisp to the touch. Turn off the oven and let them cool inside with the door slightly ajar for 30 minutes.
7. Remove from Pans: Once the meringue layers are fully cooled, gently peel them off the parchment paper.

Prepare the Chocolate Hazelnut Filling:

8. Make the Ganache: Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer. Remove from heat and add the chopped dark chocolate. Stir until smooth and melted.
9. Add Hazelnut Spread: Stir in the hazelnut spread and powdered sugar, mixing until well combined. If you want a creamier texture, stir in butter at this stage. Let the filling cool slightly before assembling the cake.

Assemble the Cake:

10. Layer the Cake: Place one meringue layer on a serving platter. Spread a generous amount of the chocolate hazelnut filling over it.
11. Top with Second Meringue Layer: Carefully place the second meringue layer on top of the filling.
12. Garnish: Decorate the top with whole hazelnuts and shaved chocolate for a beautiful finish.