To make the cinnamon apples, preheat the oven to 350 F.
Heat an 11-12” cast iron or oven-safe skillet over medium heat and add the apples, butter, brown sugar, and cinnamon.
Cook, stirring occasionally, until the apples are just starting to get tender and release some juice, about 10 minutes. You’re looking for a nice syrupy consistency.
Remove from the heat and set aside.
To make the cake, in a medium bowl, whisk together the flours, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a large mixing bowl, whisk together the butter, sugar, and applesauce.
Then whisk in the vanilla and eggs until well combined.
Stir in the dry ingredients until almost combined. The batter will be thick, so halfway through mixing, add the milk and stir until just combined.
Remove 1/2 cup of the cooked apples with some of the syrup from the skillet and set aside.
Then pour the batter into the skillet with the remaining cooked apples and fold to combine the two. Don’t overmix. Just combine enough that the apples are distributed through the batter.
Top with the reserved ½ cup of apples. Sprinkle with the remaining 1 Tablespoon of brown sugar, concentrating the sugar over the apple pieces.
Bake for 40-45 minutes or until a toothpick in the center comes out clean with a few moist crumbs
Cool the cake in the skillet on a cooling rack for about 15 minutes and make the icing.
To make the icing, whisk together the powdered sugar and most of the milk. Whisk in more milk a teaspoon at a time until smooth and the consistency of glue. If it’s too thin, whisk in a spoonful of sugar at a time until you reach the perfect consistency.
Drizzle the icing over the warm cake.
Serve warm if possible, but the cake is good at room temp for up to 2 days. It also freezes well. Cool cake completely before freezing.