Beat the egg whites, add powdered sugar, beat until stiff and set aside.
Beat the butter, egg yolks, powdered sugar, vanilla sugar and vanilla flavoring until frothy. Then stir the Nutella into the mixture. Mix well, then stir in the flour, baking powder and cornstarch. The dough then becomes very firm and already yeast-like. Now carefully stir in the egg whites little by little. Now the mass has a nice, smooth consistency again.
Put in a pre-greased springform pan. Bake for about 60 minutes. Do a toothpick test. Cool down and tumble.
To refine the jam, heat it up a little and mash it through (or pour it pure on the cake, this is entirely up to you). Spread a thick layer on the cake with a knife. Heat the couverture according to the instructions and carefully pour over the jam.