Preheat oven to 350F. Line a 9-inch round cake pan with parchment paper. Spray the sides with oil. Set aside.

To make the sauce, stir together butter, brown sugar, and salt until butter and sugar are melted. Pour into the cake pan. Top with sliced apples.

In a mixing bowl, beat melted butter and sugar together. Beat in eggs, one at a time. Add milk and vanilla and beat until combined.

In a separate bowl, whisk together flour, baking powder, cinnamon, and salt until no lumps remain. Add to the butter mixture and stir until just combined. Pour the batter over the apples and use a rubber spatula to smooth it out.

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

Let cool for about 30 minutes in pan on top of wire rack. Run a knife around the outside of the cake to loose any stuck edges. Flip onto a plate. Let cool completely before dusting with powdered sugar and serving.