Peel the orange so that the white skin is completely removed. Cut the orange into about 12 slices and roll them in sugar. Dry orange slices either on the radiator or in the oven (can be prepared a few days in advance). Wash the lemon, grate dry and finely grate the peel. Whisk the lemon zest, fat, 125 g sugar, vanillin sugar and 1 pinch of salt until thick and creamy.
Stir in the eggs one by one. Mix flour, starch and baking powder. Alternately stir into the fat mass with the lemonade. Fold in sour cream. Pour the dough into a greased and floured box form (1 1/2 litre capacity) and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 50-60 minutes. Remove from the oven and let rest for 10 minutes. Turn the cake out of the tin and let it cool down. Stir jam until smooth. Cut the cake in half horizontally.
Remove from the oven and let rest for 10 minutes. Turn the cake out of the tin and let it cool down. Stir jam until smooth. Cut the cake in half horizontally. Spread the jam evenly on the bottom half. Place the upper half on top and press down a little. Decorate with orange slices and dust with icing sugar
Time for the cake (without orange slices):