Follow these steps to bring this delightful cake to life in your own kitchen:

1. Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan.
2. In a large bowl, cream together 1 cup of granulated sugar and 1/2 cup softened butter until light and fluffy, about 3 minutes.
3. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
4. In a separate bowl, whisk together 2 cups of flour, baking soda, baking powder, and salt.
5. Gradually add the dry ingredients to the wet mixture, alternating with the sour cream. Mix until just combined.
6. Fold in the diced apples gently, ensuring they are evenly distributed throughout the batter.
7. Pour the batter into the prepared pan, spreading it evenly.
8. To make the crumb topping, mix 1 cup brown sugar, 1 cup flour, melted butter, and cinnamon in a bowl until crumbly.
9. Sprinkle the crumb topping evenly over the cake batter.
10. Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
11. Let the cake cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.

Remember, the key to a perfect crumb topping lies in the texture of your crumbs—aim for a coarse, pea-sized consistency for the best results.