Place the fat in large pieces in a mixing bowl. Whisk with the whisk of the hand mixer until foamy. Add sugar, vanilla sugar and salt and stir until a creamy mixture is formed.
Separate eggs. Stir the egg yolks into the fat-sugar mixture. Wash and dry oranges and grate the peel. Squeeze 1 orange. Stir in quark, ricotta, milk, semolina, orange peel and orange juice.
Beat the egg whites until stiff and add to the curd mixture. Carefully fold in with a whisk. Put the dough into a greased, breadcrumbed pan of the oven (approx. 30 x 35 cm) and smooth it down.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: not suitable/ gas: level 2) for approx. 1 hour. If necessary, cover with parchment paper after about 40 minutes. Let the cake cool down. Dust with icing sugar before serving.