Preheat oven to 180°C (356°F) with top and bottom heat. Mix flour, butter or margarine, powdered sugar, vanilla sugar, egg yolk, and salt to form a smooth shortcrust pastry. Divide the dough in half. Wrap one half in cling film and refrigerate.
Line a baking sheet with baking paper. Roll out the other half of dough on the sheet (use a baking frame if needed for even edges). Prick several times with a fork to prevent bubbling.
Bake in the preheated oven at 180°C (356°F) for about 15 minutes until the pastry is golden yellow but not completely baked. Remove from the oven and let cool.
Meanwhile, prepare the apple filling: roughly grate 1.5–2 kg apples. In a large bowl, mix with lemon juice, sugar, cinnamon, and rum to taste. Add raisins to taste or omit.
Spread a thin layer of apricot jam on the cooled shortcrust base. Spread the apple filling on top and smooth evenly.
Roll out the second half of dough to the size of the tray and place it over the apple filling. Prick several times with a fork. Whisk the egg yolk with 2 tablespoons milk and brush over the surface. Bake in the preheated oven at 180°C (356°F) for about 45 minutes until golden.
Cool the cake, sprinkle with powdered sugar, and serve with a dollop of cream if desired. For cleaner slices, cut the cake the next day.
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