1. Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan.
2. In a medium bowl, whisk together 2 cups of flour, baking powder, baking soda, salt, and cinnamon. Set aside.
3. In a large bowl, cream together the softened butter and sugar until light and fluffy, about 3-4 minutes. This step reminds me of my mom’s ability to make magic with just a few simple ingredients.
4. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
5. Mix in the sour cream until just combined, then gradually add the dry ingredients, mixing until smooth.
6. Fold in the diced apples gently, ensuring they are evenly distributed throughout the batter.
7. Pour the batter into your prepared cake pan, smoothing the top with a spatula.
8. For the crumble, mix together the brown sugar, 1/4 cup flour, and melted butter in a small bowl until crumbly. Sprinkle this mixture evenly over the cake batter.
9. Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean. The smell will transport you to a warm, welcoming kitchen, just like it used to do for me.
10. Allow the cake to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.

Serve this cake slightly warm for that fresh-from-the-oven experience, or at room temperature for a more settled flavor profile.