Preheat oven to 350°.

Melt butter in a 10 inch cast iron skillet with brown sugar over medium heat, about 5 minutes. Stirring constantly until brown sugar is melted.

Remove from heat and top with chopped walnuts.

Peel, core and thinly slice apples. Spread apples in a single layer over walnuts.

In a mixing bowl, combine flour, sugar, brown sugar, cinnamon, baking powder and salt.

Add milk, melted butter and eggs. Mix with a wire whisk until smooth.

Pour over apples and smooth in an even layer.

Place skillet on a rimmed baking sheet to catch any overflow of the caramel layer.

Bake for 45- 50 minutes or until a toothpick inserted comes out clean.

Allow cake to cool 15 minutes.

Place a serving plate over the top of the skillet and while holding both sides, invert cake onto cake plate.

If any walnuts or caramelized sauce sticks to skillet, just carefully scrape from skillet and place on cake.

Cake can be served warm or at room temperature.

Cover and refrigerate leftovers. Best if eaten within 2 days.