Brush a 9 or 10-inch cast iron skillet with a little of the melted butter.
Core the apples and peel all but one. Slice the unpeeled apple into half moons about 1/4-inch thick. Cut the remaining apples into 1/2-inch chunks.
Whisk the flour, baking powder and salt together in a mixing bowl.
Lightly beat the eggs in another bowl with the sugar just until foamy (an immersion blender is great for this task). Stir in the whiskey, vanilla and buttermilk.
Pour half of the egg mixture over the flour mixture and fold briefly. Pour in the butter and stir. Add the remaining egg mixture and stir until no flour is visible. Add the chopped apples and stir gently to blend.
Spoon the batter into the skillet. Arrange the sliced apples on the top, then sprinkle with the turbinado sugar.
Bake until the cake springs back when poked in the center, and the edges are golden brown, 55-60 minutes. Cool 10 minutes. Dust with powdered sugar and serve warm or at room temperature.
Feel free to substitute the whiskey with another amber spirit, like bourbon or Cognac.Recipe adapted from Cook Beautiful