Preheat oven to 350F. Grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk cake flour, baking powder, and salt. Set aside
Beat butter until smooth. Add sugar and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla and mashed bananas and mix until well combined.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Do not overmix.
Spread batter evenly into prepared pans and bake for 30mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
Toast hazelnuts at 350F for 8 minutes. Immediately wrap in a tea towel and cool. Use tea towel to rub the skins off and finely chop the nuts.
Preheat oven to 250F and line a 10x15" pan with parchment.
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 into the meringue and spread evenly in pan.
Bake for 30mins until set and pale golden. Use a 6" cake ring to stamp out two rounds.** Return to oven and bake until crisp (45-60 mins). Cool in pan then remove the circles. You may need to trim them a bit and it's ok if they break!
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 no longer grainy to the touch (approx. 3mins).
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. Add Nutella and whip until smooth.***
Place chopped chocolate and cream into a microwave safe bowl. Stir to combine. Microwave for 20 seconds, stir. Microwave in 10 second intervals, stirring in between, until ganache is smooth and silky. Set aside to cool completely before using on cake.
Preheat oven to 200F. Line a sheet pan with foil and spray with cooking spray. Toss sliced bananas with lemon juice and space evenly on baking sheet. Bake for 1 hour and flip the slices over. Bake for an additional 90mins or until desired crispiness. Cool completely.
Place one layer of the cake onto a cake stand or serving plate. Top with 1/2 cup of buttercream and spread evenly. Place one meringue disc on top and top with additional buttercream if desired. 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.**** Place banana chips along the bottom. Use a teaspoon to drip the ganache along the top edges then fill in the rest of the top with the ganache.
* 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. ** If you don't have cake rings you can skip this step and just roughly crush up the meringue when placing it in the layers *** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. **** I tried to go all fancy like and added some Nutella along the sides of the cake to make a "watercolor" effect. It didn't totally work, but in case you're wondering why yours might look different! You will have some hazelnut meringue left over. You can either add more into the layers, decorate the top with it, or eat it on it's own like I do!