Preheat oven to 180°C (160°C fan) and line an 8x10-inch baking tray.
Toss diced apples with sugar and cinnamon, then set aside.
Mix brown sugar, cinnamon, and melted butter to create the swirl mixture.
In a large bowl, whisk flour, sugar, and baking powder.
Add milk, vanilla, and eggs; mix until just combined.
Stir in melted butter gently.
Fold in the apples and pour batter into the tray.
Spoon the cinnamon swirl over the top and swirl lightly.
Bake for 40–45 minutes until mostly set.
Prepare glaze by mixing icing sugar and milk until smooth.
Pour glaze over warm Cinnamon Apple Cake and let it soak in.
Cool for 45 minutes, slice, and serve.