Preheat oven to 350°F.
Using a 9” cast iron skillet, melt butter over medium heat until bubbly. Sprinkle brown sugar evenly over melted butter. Do not stir, but allow to sit over heat for just a minute until it starts to dissolve. Remove from heat.
Peel the apples, cut them in half, and remove the seeds and stem portion. Set them face down on a cutting board and cut the halves into thin slices while maintaining the original shape. Gently place these apple slices in a circular spoke pattern in the butter and brown sugar mixture.
To make the cake batter, combine the flour, sugar, baking powder, cinnamon, and salt in a medium sized bowl. Set aside.
Separately, mix together egg, buttermilk, vanilla, and applesauce. Add this wet mixture to the dry mixture and stir just to combine. Do not over mix – this will be very similar to pancake batter.
Pour over apple pieces taking care not to change their position.
Bake in preheated oven until golden brown, about 40 minutes. Remove from oven and allow to cool slightly before inverting onto plate.