Cook the Apples
In a large cast iron skillet over medium heat, melt the butter. Add diced apples, brown sugar, and cinnamon. Stir occasionally and cook for 8–10 minutes, or until apples are tender and syrupy. Remove from heat and set aside.

Preheat the Oven
Preheat your oven to 350°F (175°C). If using the same skillet, ensure it’s oven-safe. Grease it well if not already buttered from the apples.

Mix the Dry Ingredients
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.

Combine the Wet Ingredients
In a large bowl, mix the melted butter and brown sugar until smooth. Whisk in applesauce, vanilla extract, and eggs until fully incorporated.

Finish the Batter
Add the dry ingredients to the wet in two parts, alternating with the milk. Stir until just combined avoid overmixing. Fold in most of the cooked apples, reserving about ½ cup for topping.

Bake the Cake
Pour the batter into the skillet and spread evenly. Top with reserved apples and sprinkle with 1 tbsp of brown sugar. Bake for 40–45 minutes, or until a toothpick inserted in the center comes out clean.

Make the Icing
While the cake cools for 15 minutes, whisk together powdered sugar and milk until smooth. Adjust consistency with more sugar or milk as needed.

Glaze and Serve
Drizzle the icing over the warm cake. Slice and serve warm or at room temperature.