1. Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking pan.
2. Mix together the flour, light brown sugar, granulated sugar, ground cinnamon, and salt in a medium bowl.
3. Pour the melted unsalted butter into the dry topping mixture and stir until small crumbs are formed; set aside.
4. In a separate large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt to create the dry ingredients mixture for the cake batter.
5. In another bowl, beat together melted butter, milk or sour cream, vegetable oil, eggs, and vanilla extract to form the wet ingredients.
6. Gradually mix the wet and dry cake batter ingredients together until smooth and just incorporated.
7. Spread the prepared cake batter evenly into the greased baking pan.
8. Peel and slice your apples, toss them with lemon juice, granulated sugar, and cinnamon, and layer them evenly on top of the cake batter.
9. Sprinkle the prepared crumb topping generously over the apple layer, pressing it slightly to help the crumbs adhere to the cake.
10. Place the assembled cake into the preheated oven and bake for 30-35 minutes.
11. Check for doneness by inserting a toothpick into the cake layer; it should come out clean or with light crumbs.
12. If the topping browns too quickly while baking, cover the pan loosely with aluminum foil.
13. Remove the cake from the oven and let it cool in the pan on a wire rack.
14. Slice and serve your Apple Crumb Cake as is, or pair it with whipped cream, vanilla ice cream, or a drizzle of caramel glaze for additional flair.