Preheat your oven to 350°F (175°C). Prepare a 9x13 inch baking pan by greasing it lightly with butter or cooking spray.

In a large bowl, combine the flour, baking powder, baking soda, ground cinnamon, and salt. Stir well to ensure they are evenly mixed.

In another bowl, cream the softened butter and granulated sugar together until the mixture is light and fluffy. This should take about 3–5 minutes.

Incorporate the eggs one at a time, mixing well after each addition. Add the vanilla extract and continue mixing until thoroughly combined.

Alternately add the dry ingredients and sour cream to the butter mixture, starting and ending with the dry mix. Mix just until combined—overmixing can make the cake dense!

Gently fold in the chopped apples, ensuring they are evenly distributed throughout the batter.

In a separate bowl, combine brown sugar, rolled oats, ground nutmeg, and remaining flour. Add melted butter or softened butter and mix until crumbly.

Pour the batter into the prepared baking pan, spreading it evenly. Top with the crumb mixture, ensuring an even layer over the batter.

Place the cake in the preheated oven and bake for about 50-60 minutes, or until a toothpick inserted in the center comes out clean.

Once baked, allow the cake to cool in the pan for at least 15 minutes before slicing.

Enjoy warm or at room temperature, and watch your family devour it!