Preheat oven to 350F, grease three 6" round baking pans and dust with cocoa powder. Line bottoms with parchment.
Place all dry ingredients into the bowl of a stand mixer fitted with a paddle attachment. Stir to combine.
In a medium bowl whisk all wet ingredients (pour hot water in slowly while whisking as not to cook the eggs).
Add wet ingredients to dry and mix on medium for 2-3 mins. Batter will be very thin.
Pour evenly into prepared pans. I used a kitchen scale to ensure the batter is evenly distributed.
Bake until a cake tester comes out mostly clean, a total of 30-35mins.
Prepare a stand mixer with a whisk attachment and whip butter until creamy (2 mins).
In a medium bowl, whisk together sifted powdered sugar and cocoa powder.
Reduce speed to low and add in powdered sugar mixture 1 cup at a time until well blended. Scrape down sides of bowl and whisk as needed. Add in Nutella and whip on high for approximately 5 mins.
Preheat oven to 250F and line a 10x15" pan with parchment. Trace two 5" circles onto the parchment, flip over so the traced side is down.
Combine egg whites, salt, and cream of tartar in the bowl of a stand mixer**. Beat on high until soft peaks form. Slowly add sugar (1 Tbsp at a time) and beat until stiff peaks.
Dab a bit of meringue under the corners of the parchment to secure it. Fold chopped hazelnuts and cocoa powder into the meringue. Pipe into discs using the 5" circles as a guide. Pipe or spread remaining meringue in empty spaces on the parchment.
Bake for 90mins and cool on pan.
Place one layer of the cake onto a cake stand or serving plate. Top with 2/3 cup of buttercream and spread evenly.
Place one meringue disc on top and top with additional buttercream if desired (or sprinkle with chopped Ferrero's. Repeat with remaining layers and crumb coat the cake. Chill for 20mins.
Frost the cake with the remaining buttercream, smooth the sides and chill for 20mins. Press chopped hazelnuts into the sides and top with rosettes if desired.
* Grind up 1 cup toasted, skinned hazelnuts to a fine powder. Or sub in 3/4 cup all-purpose flour.** Ensure there is no trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen.