Step 1: Preheat your oven to 350°F (175°C) and grease a 9-inch square baking pan or line it with parchment paper.
Step 2: In a medium bowl, prepare the crumb topping by mixing 1 cup flour, brown sugar, and 2 teaspoons of cinnamon. Cut in the 1/2 cup of cold butter until the mixture resembles coarse crumbs, then refrigerate.
Step 3: In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Add the eggs one at a time, beating well after each addition, then stir in the vanilla and sour cream.
Step 4: Whisk together the 2 cups of flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
Step 5: Gently fold the chopped apples into the batter and spread the mixture evenly into the prepared baking pan.
Step 6: Sprinkle the chilled crumb topping generously over the batter, covering the entire surface.
Step 7: Bake for 45-55 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool for at least 15 minutes before slicing and serving your apple cinnamon crumb cake recipe.