Preheat the oven at 170°C and butter a round pan with a removable base of 24cm.
For the base: In a small pot, add the soft light brown sugar, the unsalted butter, the cinnamon and put it on the heat. Boil until the sugar dissolves. Reduce the heat at medium and heat it up for 2-3 minutes. Add the salt, stir and place the mixture in the pan's base. Peel the apes, remove the seeds and cut in slices. Place them in the pan, close enough to touch each other, covering the whole pan. Cut the remaining apples into small pieces and add them in the cake later.
Prepare the cake: In a bowl, mix the flour, baking powder, cinnamon, salt, vanillin sugar and walnuts. In the mixer's bowl place the unsalted butter, sugar, and soft light brown sugar. Mix for 2-3 minutes, until they fluff up. Add the eggs one by one and continue mixing until we have a fluffy mixture. Reduce the mixer's speed and add half the flour amount spoon by spoon. Add the milk and the rest of the flour. Add the remaining apple pieces and remove the bowl from the mixer. Stir with a spatula and place the cake mixture on top of the apples that are in the pan. Bake in a preheated oven for 40 minutes. Set it aside for 15 minutes to cool down and serve. If we like, we can add a scoop of our favourite ice cream flavour to each piece.