Biscuit preparation: Place egg whites in a bigger bowl and mix with hard mixture until stiff, then add sugar little by little until melted. Stir cacao and hazelnut powder and mix well with spatula. Preheat the oven on 180C. Place baking sheet into the baking tray, add the mixture and lay it evenly.
Bake biscuit for 35 minutes and leave it to cool down.
Yellow Cream preparation: Mix egg yolks with sugar using hand mixer. Add vanilla sugar and flour and keep mixing.
Put milk in a pot and heat it on the stove ( low-medium heat). Whisk in the cream slowly. Whisk constantly until the cream is thickened (around 10min). Put it aside and wait till it cools down completely. Stir it occasionally.
Chocolate Cream preparation: Mix eggs with sugar until sugar is melted well. Put it in a bowl (double boiler method) and cook on the steam (over low-medium heat), crush chocolate and add it to the mixture. Stir well until the cream is thickened. Put it aside to cool down. When cooled down add softened butter and stir well with hand mixer. Cool it down. (in the fridge).
Glaze preparation: Crumble the dark chocolate, add oil and cook on low heat (double boiler method) until melted. Cool down.
Layers: When biscuit and cream are cooled down cut the biscuit into 3 even layers. Put one a top another and cut the ends (rims) if necessary (in order to be even).
On a cake serving plate place one layer first. Spread yellow cream evenly on the layer and then spread chocolate cream on top of the yellow one.
Put another layer and repeat the process. You should use all the yellow cream and still have chocolate cream left for decoration.
Decoration: With the remaining chocolate cream decorate sides of the cake first (with cake decorator, syringe). Make sure all sides are well covered. On the top of the cake decorate just the angle and trims. Then carefully pour the chocolate glaze in the middle, spread it evenly with a spoon.
For additional decoration on the top you can use chopped nuts or decorate as you wish.