Peel and quarter the apples and remove the core. Cut the quarter into cubes. Cook the cubes and 50 g sugar in a saucepan over medium heat for about 20 minutes to a chunky compote. Let cool off.
Bring the milk and 100 ml advocaat to the boil. Stir pudding powder, 2 tablespoons sugar and 50 ml advocaat until smooth. Stir into the boiling milk liqueur mixture, bring to the boil and simmer for about 1 minute. Remove from the heat, pour into a bowl and cover with cling film.
Leave the pudding to cool for at least 1 hour.
Roast the walnuts in a pan without fat. Cream butter, 200 g sugar, salt and vanillin sugar with the whisks of the hand mixer. Separate 6 eggs, stir egg yolks and 1 egg one after the other into the butter-sugar mixture.
Mix flour and baking powder and stir in alternately with the quark. Fill into a greased deep baking tray (34 x 39 cm), dust with flour and smooth it down.
Mix compote and pudding together and spread on the dough. Finely grind the walnuts. Beat the egg whites until stiff, then pour in 100 g sugar. Stir in ground nuts and spread on the apple compote.
Bake in the preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see manufacturer) for approx. 1 hour 10 minutes, cover with aluminium foil after approx. 20 minutes of baking time. Remove from the oven, let cool and cut into pieces.