Cream butter, sugar, 1 pinch of salt, vanilla sugar and orange juice with the whisk of the hand mixer. Separate the eggs. Stir egg yolks and yoghurt into the butter mixture.
Mix nuts, speculoos spice and baking powder and stir in. Beat the egg whites with the whisks of the hand mixer until stiff. Carefully fold the egg white into the nut mixture.
Put 2 tablespoons of dough into a greased, floured box form (11 x 21 cm) and smooth it down, add Nutella, add the remaining dough and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 45 minutes.
Remove from the oven, let it cool down for about 10 minutes, turn out on a cake rack and let it cool down. Dust with icing sugar if necessary.