2) In a medium bowl, whisk together the eggs, liqueur and hazelnut essence until blended and set aside. In a large bowl of an electric stand mixer, gently combine on low speed the 500g flour, sugar, baking powder, bicarbonate of soda, salt and hazelnuts until blended. Add the butter and sour cream and beat on a low speed until the dry ingredients are moistened.
3) Increase speed to medium and beat for 2 1/2 minutes, scraping the sides of the bowl occasionally. In three separate additions, add the egg mixture to the batter, beating for 30 seconds after each addition. Divide the batter evenly into the prepared tins and bake on the centre rack for 40 to 45 minutes or until a cake tester inserted into the centre of each cake comes out clean. Cool the cakes in the tins for 10 minutes then remove from the tins, discard the greaseproof paper and cool completely on wire racks, 1 to 2 hours.
4) To make the hazelnut filling, in a medium saucepan melt the butter over medium heat. Sprinkle with flour and whisk until smooth.
5) Reduce the heat to medium-low, add the sugar and cream and cook, whisking constantly, for 7 to 10 minutes or until the mixture is thickened. Stir in the hazelnuts, salt, liqueur, hazelnut essence and chocolate, and cook just until chocolate melts. Set aside in a bowl and cool completely.
6) To make the chocolate buttercream icing, melt the 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 the mixture reaches 60C.
7) Remove the 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 the mixture is cool to the touch and doubled in volume. Reduce speed to low and add the softened butter 1 tbsp 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 essence and beat until fluffy.
8) To assemble the cake, split each cake layer horizontally in half to make four separate layers. Spread equal amounts of the hazelnut filling between each layer and spread buttercream icing over the top and sides of the cake. 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 icing. Place whole hazelnuts and chocolate curls on the top of the cake.
Cooks Notes: 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 the nuts in 480ml water along with 2 tbsp bicarbonate of soda for 1 to 2 minutes. Run the nuts under cold water and slip off skins. Dry the nuts well and toast on a baking sheet in a 180C/Gas 4 oven for 20 minutes, stirring occasionally. Cool, finely chop and proceed with recipe. If you purchase the nuts already peeled, toast in a 180C/Gas 4 oven for only 15 minutes.
I overlap cooking times (the filling can be made while the cake cooks, for example, and the icing can be fully prepared while the cake cools).
A viewer, who may not be a professional cook, provided this recipe. The Food Network Kitchens chefs have not tested this recipe and, therefore, we cannot make representation as to the results.