Peel and quarter the apples and remove the core. Cut quarters into large cubes. Boil up the apple juice in a large pot. Add apples and cover and cook for about 5 minutes. Mix pudding powder with 1 tbsp. sugar and stir with advocaat until smooth.
Pull the pot from the stove, stir in the pudding powder. Simmer for about 1 minute while stirring. Remove from the heat, pour into a bowl and allow to cool.
Roast the walnuts in a pan without fat, let them cool down. Cream butter, 250 g sugar and salt with the whisk of the hand mixer. Separate the eggs, stir the egg yolks one after the other into the butter-sugar mixture.
Mix flour and baking powder and stir in alternately with the quark. Pour into the greased, flour-dusted fat pan of the oven (34 x 39 cm) and smooth it down.
Stir the compote briefly, spread on the dough. Finely grind the walnuts. Beat the egg whites in 2 portions until stiff, adding 175 g sugar to each portion. Fold the ground nuts into the beaten egg white and spread loosely on the apple compote.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 35-40 minutes on the lower rack. Remove from the oven, allow to cool on a cake rack and cut into pieces.