Preheat the oven to 350°F (175°C).
In a 9-inch round cake pan, melt the butter over low heat. Once melted, remove from heat and sprinkle the brown sugar evenly over the melted butter.
Arrange the apple slices on top of the brown sugar mixture in a decorative pattern. Set aside.
In a bowl, whisk together the flour, baking powder, salt, and cinnamon.
In a separate large bowl, cream together the granulated sugar and eggs until light and fluffy. Add the vanilla extract and mix well.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Mix until just combined.
Pour the batter over the arranged apples in the cake pan, spreading it evenly.
Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, then invert onto a serving plate. Serve warm or at room temperature.