Preheat oven to 175°C (350°F). Grease a 9-inch cake pan.
Make the apple topping: In a pan, melt butter and brown sugar over medium heat. Stir in cinnamon, then pour into the cake pan. Arrange apple slices on top.
Make the cake batter: Whisk flour, baking powder, cinnamon, and salt. In a separate bowl, beat butter and sugar until fluffy. Add eggs, vanilla, and mix. Alternately add dry ingredients and milk until smooth.
Assemble and bake: Pour batter over apples and spread evenly. Bake for 35–40 minutes, or until a toothpick comes out clean.
Cool & flip: Let cool for 10 minutes, then invert onto a plate. Serve warm!
