Preheat oven to 350°F (175°C). Grease and line an 8-inch round cake pan.

Prepare topping: In a skillet, melt butter over medium heat. Stir in brown sugar and cinnamon until dissolved. Add sliced apples and cook until slightly tender, about 5–7 minutes.

Pour caramelized apples into the prepared cake pan; set aside.

Make cake batter: In a bowl, whisk flour, baking powder, and salt. Separately, cream butter and sugar until fluffy. Add eggs one at a time, then vanilla, mixing well.

Alternate adding dry ingredients and buttermilk into the butter mixture, mixing gently until just combined.

Pour cake batter evenly over caramel apples.

Bake for 30–35 minutes, until a toothpick comes out clean.

Cool for 10 minutes, then carefully invert the cake onto a serving plate.