For the apple sheet cake, peel the apples, remove the core and cut the flesh into small pieces. Line a baking tray with baking paper and preheat the oven to 180 degrees Celsius.
Beat the eggs with the sugar, vanilla sugar and a pinch each of salt and turmeric. Mix the oil, milk and skyr.
Mix the flour with the baking powder and cornstarch and stir into the egg mixture in portions. Spread the dough on the baking sheet.
Spread the apple pieces evenly on the dough and bake the apple sheet cake at 180 degrees convection for about 30 minutes.