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 water in slowly as not to cook the eggs if very hot).
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 35-40mins.
Cool 10 minutes in the pans then turn out onto a wire rack to cool completely.
Nutella Swiss Meringue Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a hot water bath on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth and fluffy.** Add Nutella and mix until incorporated and buttercream is smooth and fluffy.
Place Nutella and cream in a microwave safe bowl. Microwave in 10 second increments, stirring in between, until completely smooth. Strain if needed to ensure it's completely smooth (I did).
Allow to cool completely and thicken before using on cake.
Cut each layer of cake in half horizontally to create 6 total layers.
Place one layer of cake on a cake stand or serving dish. Top with about 1/2 cup buttercream. Drizzle with some of the ganache (1-2Tbsp).*** Sprinkle chopped hazelnuts on top (optional). Repeat with remaining layers. Frost and smooth the outside with a thin crumb coat. Chill for 20mins.
Frost and smooth the outside of the cake, generously frosting the sides specifically. Smooth with an icing smoother. Use a cake comb on the sides of the cake. Repeat, scraping off more frosting each time, until smooth.
Using a small spoon, place dollops of ganache around the top edges of the cooled cake, allowing some to drip down. Fill in the top of the cake with more ganache and spread evenly with an offset spatula.
Pipe rosette dollops on top using a mix of tips 6B, 4B, and 32.
* 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. ** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. *** Be careful not to drizzle too much of the ganache between the layers as it can cause them to slide around. It's just to add a bit more flavor.Nutella Ganache recipe adapted from In Good Taste.