Prepare the oven and pan: Preheat your oven to 350°F. Grease a baking dish well so the cake lifts easily later. This step helps keep the edges soft and prevents sticking.
Mix the cake batter: In a large bowl, add the cake mix, flour, brown sugar, and salt. Stir gently to combine. Add water, oil, and eggs, then mix until smooth. The batter should look thick yet pourable.
Pour the base layer: Pour the batter into the prepared baking dish. Use a spatula to spread it evenly so the cake bakes level.
Prepare the apples: Place sliced apples in a bowl. Add lemon juice, brown sugar, and cinnamon. Toss until the apples look glossy and evenly coated. This keeps them bright and full of flavor.
Add the apple layer: Spread the apple mixture evenly over the cake batter. Press them down lightly so they settle into the batter while baking.
Finish with butter topping: Scatter the butter pieces across the top. As they melt, they create a rich, crumbly finish that feels like pie topping.
Bake until golden: Place the dish in the oven and bake for 45 to 55 minutes. The top should look golden and set, with soft apples underneath. Let it cool slightly before slicing.
