2. Beat the butter in a mixing bowl with an electric mixer on high speed for 30 seconds. Gradually add the sugar and continue to beat until the mixture is pale yellow and very fluffy.
3. Add the eggs one at a time while mixing on low speed. Stir in the vanilla.
4. Sift the flour, baking powder, cinnamon and salt together.
5. Add half of the flour mixture and blend, then add the milk. Finally, add the rest of the flour mixture. The batter should be smooth. Do not overmix.
6. Coat a 9" square cake pan with nonstick spray. Pour the batter into the pan. Arrange the apple slices on top in a single layer.
7. Bake until golden brown and set in the center, about 25 to 30 minutes. Serve warm.