1. Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish or line with parchment paper.

2. In a medium bowl, combine 1 cup flour, brown sugar, and cinnamon. Add 1/2 cup melted butter and mix until large crumbs form. Set aside.

3. In a large bowl, cream together remaining softened butter and granulated sugar until light and fluffy.

4. Add eggs one at a time, mixing well after each. Stir in vanilla extract and sour cream.

5. In a separate bowl, whisk remaining flour, baking powder, baking soda, and salt.

6. Gradually add dry ingredients to wet mixture. Mix until just combined.

7. Pour the batter into the prepared pan and spread evenly.

8. Sprinkle the cinnamon crumble topping over the batter generously.

9. Bake for 35–40 minutes or until a toothpick inserted in the center comes out clean.

10. Cool completely before serving. Optionally drizzle with powdered sugar glaze or cream cheese icing.