In a medium mixing bowl start mixing egg whites plus a pinch of salt with a mixer of choice. When the mixture becomes frothy (soft stiff phase), add sugar (slowly), and continue mixing. Add vinegar and mix a few more minutes until ingredients are whipped into stiff peaks. Turn the mixer off and remove from the bowl.

Add hazelnuts to the bowl and stir with a wooden spoon until batter integrates. Heat oven to 320°F (160°C). 

Line a baking pan (approximately 13x17 inches, or 33x43cm) with parchment paper. Leaving about ½ an inch of space in between layers, use the batter to create three equal rectangles the thickness of about 1-inch. (If it proves too hard to keep batter separated you can makeshift dividers out of foil. Check out the video for visual.)* 

Place in the oven and bake (or rather dry) on 320°F for 1 hour.  Check the cake every 15 minutes or so, and turn the pan 180° about 30 minutes in. Depending on the strength of your oven, you may have to cover the cake with foil if it starts to blush early. (Meanwhile, prepare the frosting and filling).

Set aside to cool completely. Once cooled, carefully separate cake from the parchment paper so that it doesn't break. 

Make a double boiler (water bath or bain marie). Fill a large pot ⅓ with water and bring to boil on high. Lower to medium and place a smaller pot with egg yolks into the large pot. (The smaller pot should be touching the water in the larger pot.) Add confectioner's sugar and water to yolks and start mixing with a hand mixer. Mix for several minutes.

When eggs and sugar integrate, turn pale yellow (almost white) and start separating from the pot walls, add in baking chocolate. Continue mixing over double boiler. Once completely integrated, turn the stove off and take the smaller pot out of the larger. Set aside to cool for 10 minutes.   

Add butter into the bowl and mix until integrated. Remove the mixer and cool in the fridge for about 20 minutes.  

Place the first cake third onto a long cake platter. Empty ¼ of frosting and filling onto it and spread evenly with a spatula. Lightly place the second cake third on top of the filling. Empty ¼ of the frosting and filling onto it and spread evenly with a spatula. Repeat for the third cake piece.

Use the remaining ¼ of the frosting and filling to paint the cake all sides.

(Optionally) Decorate cake with a few handfuls of ground or chopped hazelnuts or grated baking chocolate on top. 

Leave cake in the fridge to stiffen for at least 2 hours, preferably 4.