Preheat the Oven: Preheat your oven to 180°C (350°F). Grease a cake pan and set it aside.
Mix Dry Ingredients: In a large mixing bowl, combine the flour, baking powder, ground cinnamon, and salt. Whisk well to ensure even distribution.
Combine Wet Ingredients: In another bowl, mix together the granulated sugar, melted butter, eggs, and milk until smooth and well blended.
Combine Mixtures: Gradually pour the wet mixture into the dry ingredients, mixing gently until just combined. Be careful not to overmix.
Add Apples: Pour the batter into the prepared cake pan and gently fold in the thin apple slices, distributing them evenly.
Prepare Topping: In a small bowl, combine brown sugar, ground cinnamon, and melted butter. Drizzle this mixture over the top of the batter.
Bake: Place in the oven and bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean. Let it cool for a few minutes before serving.