Preheat your oven to 350°F (175°C).

In a mixing bowl, combine the melted butter and brown sugar. Stir until well blended and smooth. Pour this mixture into the bottom of the prepared 9-inch round cake pan, spreading it evenly.

Arrange the apple slices over the butter and brown sugar mixture in a circular pattern, sprinkling the cinnamon evenly over the apples.

In another mixing bowl, combine the flour, granulated sugar, baking powder, and salt. Mix well.

In a separate bowl, whisk together the milk, eggs, and vanilla extract. Slowly mix the wet ingredients into the dry ingredients until just combined. Ensure not to overmix.

Pour the batter over the arranged apple slices in the cake pan, spreading it gently to cover the apples completely.

Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean.

Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, carefully invert the cake onto a serving platter.

Allow the cake to cool slightly before slicing and serving.