Caramelize Apples: In a skillet, melt butter over medium heat. Add brown sugar and cinnamon, stirring until dissolved. Arrange apple slices in the caramel mixture and cook for 3-5 minutes. Remove from heat.
Prepare Batter: In a bowl, mix flour, sugar, baking powder, milk, oil, egg, and vanilla until smooth.
Assemble: Pour batter over the caramelized apples in the skillet.
Cook: Preheat oven to 350F. Cook on 350F for 20-25 minutes until a toothpick inserted into the center comes out clean.
Invert & Serve: Let cool for a few minutes, then carefully invert onto a plate.
