To make the cake, grease 2 (9 by 2-inch) round cake pans with shortening and line bottoms with parchment rounds. Grease parchment and dust each pan with 1 tablespoon flour, shaking out excess. Set pans aside.
Preheat oven to 350 degrees F.
In a medium bowl whisk together the eggs, liqueur, and hazelnut extract until blended and set aside.
In a large bowl of an electric stand mixer, gently combine on low speed 3 1/3 cups flour, sugar, baking powder, baking soda, salt and hazelnuts until blended. Add the butter and sour cream and beat on low speed until dry ingredients are moistened.
Increase speed to medium and beat for 2 1/2 minutes, scraping the sides of the bowl occasionally. In 3 separate additions, add the egg mixture to the batter, beating for 30 seconds after each addition. Divide batter evenly into the prepared pans and bake on the center rack for 40 to 45 minutes or until a cake tester inserted into the center of each cake comes out clean. Cool cakes in pans for 10 minutes. Remove from pans, discard parchment and cool cakes completely on wire racks, 1 to 2 hours.
To make the hazelnut filling, in a medium saucepan melt the butter over medium heat.
Sprinkle with flour and whisk until smooth. Reduce heat to medium-low, add the sugar and cream and cook, whisking constantly, for 7 to 10 minutes or until mixture is thickened. Stir in the hazelnuts, salt, liqueur, hazelnut extract, and chocolate and cook just until chocolate melts. Set aside in a bowl and cool completely.
To make the chocolate buttercream frosting, melt chocolate in the top of a double boiler over medium heat until smooth. Set aside, off the heat, and cool completely. In a large bowl whisk together the sugar and egg whites until blended. Place the bowl over a saucepan of simmering water and whisk until mixture reaches 140 degrees F.
Remove mixture to a large bowl of a stand mixture with a whisk attachment and beat on low speed for 1 minute to cool slightly. Increase speed to high and beat for 4 minutes or until mixture is cool to the touch and doubled in volume. Reduce speed to low and add the softened butter 1 tablespoon at a time, beating well after each addition, until all of the butter has been added. Continue to beat until mixture looks smooth. Fold in the melted chocolate and the hazelnut extract and beat until fluffy.
To assemble the cake, split each cake layer horizontally in half to make 4 separate layers. Spread equal amounts of the hazelnut filling between each layer and frost top and sides of cake with the buttercream. To garnish, press chopped hazelnuts onto the sides of the cake. Using a pastry bag fitted with a star tip, pipe rosettes around the top rim using remaining frosting. Place whole hazelnuts and chocolate curls on the top of the cake.
Tip: To make this cake an easier undertaking, take the time to search out hazelnuts that have already been peeled. If peeled nuts are unavailable, working in batches, boil nuts in 2 cups water along with 2 tablespoons baking soda for 1 to 2 minutes.
Run nuts under water and slip off skins. Dry the nuts well and toast on a baking sheet in a 350 degree F oven for 20 minutes, stirring occasionally. Cool and finely chop and proceed with recipe. If you purchase the nuts already peeled, toast in a 350 degree F oven for only 15 minutes.
I overlapped cooking times (the filling can be made while the cake cooks, for example, and the frosting can be fully prepared while the cake cools). A viewer, who may not be a professional cook, provided this recipe