For the dough, warm milk lukewarm. Dissolve yeast in it. Melt the fat. Put flour and sugar in a mixing bowl. Add yeast milk, fat and egg. Mix to a smooth dough with the dough hooks of the hand mixer.
Cover and leave to rise in a warm place for about 30 minutes. Meanwhile, wash apples for the topping, cut them into quarters and cut out the core. Cut the fruit flesh into slices and sprinkle with lemon juice. Halve the walnut halves. Mix apples, nuts and 50 g sugar. Roll out the dough on a greased deep baking tray. Cover and leave to rise in a warm place for another 30 minutes. Spread the apple mixture on top. Bake in the preheated oven (electric cooker: 175 °C/ gas: level 2) for 12 minutes. Whisk eggs, sour cream and remaining sugar. Spread over the cake.
Roll out the dough on a greased deep baking tray. Cover and leave to rise in a warm place for another 30 minutes. Spread the apple mixture on top. Bake in the preheated oven (electric cooker: 175 °C/ gas: level 2) for 12 minutes. Whisk eggs, sour cream and remaining sugar. Spread over the cake. Bake for another 30-35 minutes. Serve dusted with icing sugar