Grandissimo cake - big, rich cake for all occasions

Viewers went crazy for this Grandissimo cake, and we had to make a video recipe.
Written recipe at: domacirecepti....
Measures for one crust, it is necessary to repeat 4x:
4 egg whites
5 tablespoons sugar
50 ml cold water
2 full tablespoons cocoa
1 tablespoon soft flour
5 g or half a bag of baking powder
4 full tablespoons roasted chopped or blended hazelnuts.
Filling 1, 2 and 3:
1.5 liters of milk
16 yolks
12 tablespoons sugar (or to taste)
10 tablespoons soft flour
5 tablespoons starch
375 g butter (3 packs of 125 grams)
150 + 50 g of chocolate
100 g of roasted ground hazelnuts
1 teaspoon instant coffee (for children's versions white chocolate or Nutella)
1 tablespoon hot water
More: 1/2 liter sweet sour cream
Decoration as you wish