Cream icing sugar, fat and salt with the whisk of the hand mixer. Stir in the eggs one after the other. Mix flour and baking powder and stir in alternately with 150 ml orange juice.
Fold in the almonds. Pour the dough into a greased, square springform pan (24 x 24 cm) dusted with flour and smooth it down. Bake the dough in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 50 minutes.
Take the cake out of the oven and let it cool down on a cake rack. Stir the mascarpone, quark and 4 tablespoons of orange juice until smooth. Mix cream firmer and sugar and stir into the cream. Wash and dry apples.
Grate the flesh from 1 apple around the core. Immediately fold into the mascarpone mixture. Halve the cake once horizontally. Spread the mascarpone mixture evenly on the base and smooth it down.
Put the lid on it. Cut the cake into 16 cubes. Peel oranges so that the white skin is completely removed. Cut oranges and 1 apple into slices. Sprinkle with lemon juice. Cover cake pieces with orange and apple slices.
Mix grape juice and icing powder. Bring to the boil and sprinkle the fruit with it.