First make caramel hazelnuts: on low heat dissolve sugar, let it caramelize. Roughly crush hazelnuts and add to caramelized sugar, stir to coat, remove from heat. Spread on an oiled pan or parchment paper, let cool and then roughly grind.
For the filling, prepare gelatin according to package instructions. Cook milk with vanilla essence and sugar until it boils. Remove from heat and add dissolved gelatin, stir and let cool at room temperature. Then put in fridge and let cool completely. You should get the consistency of pudding or panna cotta.
For the bottom layer, mix egg yolks with butter. Add caramel hazelnuts and sifted. Spread over the bottom of springform pan (26 cm) lined with parchment paper and bake for 10-15 min. at 180 Celsius.
For middle layers, whip egg whites until stiff peaks form. Gently add caramel hazelnuts and divide the batter in two. Spread each part of over the bottom of springform pan (again 26 cm) lined with parchment paper and bake for 30 min. at 120 Celsius.
When the filling is completely chilled, beat it with electric mixer. In a separate bowl beat whipping cream and then add to the milk mixture, mix the two .
Assemble: Put bottom layer on a serving plate. Put the ring of the springform pan around it (before you do that, cover the ring with plastic wrap). Put 1/3 of the filling over the bottom layer, cover with 1 inner layer. Pour another part of the filling over that layer and cover it with the second inner layer. Pour the rest of the cream over it and put in the fridge overnight. Remove the ring of the pan the next day. Whip cream and decorate cake with it. Sprinkle with some hazelnuts.