For the marble cake, separate the eggs and beat the egg whites with a pinch of salt until stiff. Beat the egg yolks with the powdered sugar and vanilla sugar until foamy. Add the hazelnut oil and mix again briefly. Mix in the egg liqueur and wholemeal spelt flour.
Divide the mixture and stir the Nutella into one half of the dough mixture. Pour a dash of mingeral water into the Nutella mixture and stir briefly.
Divide the beaten egg whites between the two dough masses and fold them in carefully. Pour the mixture alternately into a greased and floured springform pan (diameter 20 cm).
Bake at approx. 170 °C and allow to cool.