1. Preheat your oven to 350°F (175°C) and grease a 9×9-inch baking pan.
2. In a large mixing bowl, combine 2 cups of flour, 1/2 cup granulated sugar, 1/2 cup brown sugar, baking powder, baking soda, salt, and cinnamon. Stir until well mixed.
3. Add the melted butter, eggs, vanilla extract, and buttermilk to the dry ingredients. Mix until just combined. Be careful not to overmix, as this can make the cake dense.
4. Fold in the diced apples gently, ensuring they are evenly distributed throughout the batter.
5. Pour the batter into the prepared baking pan, smoothing the top with a spatula.
6. For the crumble topping, combine 1/4 cup flour, 1/4 cup brown sugar, and cold cubed butter in a small bowl. Use your fingers or a pastry cutter to mix until the mixture resembles coarse crumbs.
7. Sprinkle the crumble evenly over the cake batter.
8. Bake for 40-45 minutes or until a toothpick inserted into the center comes out clean.
9. Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Pro Tip: Letting the cake cool slightly before serving will help the flavors meld beautifully. But if you’re like me and can’t resist, a warm slice with a scoop of vanilla ice cream is pure bliss!