PREHEAT OVEN & PREPARE PAN - Preheat oven to 350° F. Brush the baking dish with butter or line with parchment paper.
PREPARE APPLES - Peel, core, and cut apples into 1/4" chunks.
MIX APPLE FILLING - Mix brown sugar, cinnamon, and flour together in the medium bowl. Then add the chopped apples. Let the apple filling sit while mixing up the cake batter.
MIX CAKE BATTER - Mix together the flour, baking powder, sugar, salt, and cinnamon in the large bowl. Add in the eggs, melted butter, milk, and vanilla. Mix until the cake batter is smooth.
ASSEMBLE CAKE - Add half of the cake batter to the prepared baking dish. Then add half of the apple filling on top of the cake batter. Repeat with the rest of the cake batter and apple filling.
BAKE - 40-45 minutes or until the top of the cake is golden brown and tooth pick comes out clean when inserted into the center of the cake. NOTE: bake times can differ depending on what type of baking dish is used. I used a cast iron skillet.
COOL CAKE - Cool the cake on a wire rack for 10 minutes.
MIX ICING - Mix the icing in the small bowl while the cake cools slightly. Add the milk 1 tablespoon at a time until the right consistency is achieved.
SERVE IMMEDIATELY - Serve the cake warm.