Preheat oven to 350°F. Melt 1 tbsp butter in a 10-inch ovenproof skillet and swirl to coat.
Toss apple slices with 1/4 cup brown sugar and spread in skillet.
Beat remaining butter with granulated and 1/4 cup brown sugar. Add egg and vanilla; mix.
Whisk flour, baking powder, and salt. Add milk and dry mix to butter mixture until just combined.
Spoon batter over apples and smooth. Drizzle caramel sauce over batter.
Bake 30–35 minutes until top is golden and a toothpick comes out mostly clean. Let cool 10 minutes before slicing.