Whisk the softened butter and the sugar in a mixing bowl till creamy
Add the eggs one by one to mix till smooth
Lightly mix together the dry ingredients: salt, flour, baking powder, and gradually add to the mixing bowl, preferably half first, blend, then the other half, mix till the batter is smooth.
While the batter is resting, peel the apples (or leave the skin on, up to you) and slice them.
Heat the oven to 350 F or 180 C
Grease a 9" springform mold, and line the bottom with baking paper.Pour the batter into the mold, and smooth the top evenly Display the apples slices on top of the batter
Sprinkle the brown sugar mixed with the cinnamon over the apples and batter
Bake the cake at the lower shelf of the oven for 40 minutes until the top is golden. To check if the inside is done, insert a small knife or wooden stick into the cake, if it comes out clean, the cake is baked through.