Preheat the oven to 180°C and grease or line a cake tin with baking parchment.
Peel, core and slice the apples into thin wedges or small pieces and set aside.
In a large bowl, beat the eggs with the salt, vanilla sugar and granulated sugar until pale and lightly foamy.
Add the melted butter and milk to the egg mixture and whisk until smooth and combined.
Sift the flour with the baking powder, then gradually add to the wet ingredients, mixing until a smooth batter forms without lumps.
Pour about half of the batter into the prepared tin and spread it evenly across the base.
Arrange the sliced apples over this first layer of batter in an even pattern.
Pour the remaining batter over the apples, spread gently to cover and sprinkle almond flakes on top if using.
Bake for about 30 minutes at 180°C, then check with a wooden skewer; if it comes out clean, the cake is ready.
Allow the cake to cool for 15–20 minutes, then slice into portions and serve warm or at room temperature.