Preheat your oven to 350°F (175°C) and lightly grease a 9×13 inch baking dish with cooking spray or butter.
Prepare the apple filling by combining sliced apples, sugar, cinnamon, vanilla extract, lemon juice, and salt in a large bowl. If you prefer a thicker filling, add the cornstarch. Toss until all apple slices are evenly coated.
Transfer the apple mixture to your prepared baking dish, spreading it out in an even layer. The dish will look quite full, but the apples will cook down during baking.
Sprinkle the dry cake mix evenly over the apple layer, creating a complete blanket over the fruit. Do not mix the cake powder into the apples.
Pour the melted butter evenly over the cake mix, trying to cover as much of the powder as possible. Some dry spots are okay—they’ll create a nice texture contrast.
If using nuts, scatter them over the top for added crunch and flavor.
Bake for 45-50 minutes until the topping is golden brown and the apple filling is bubbling around the edges. If the top browns too quickly, loosely cover with foil.
Allow to cool for at least 15 minutes before serving to let the filling set slightly. This cobbler is delicious warm, at room temperature, or even cold from the refrigerator.