Preheat your oven to 350°F (175°C). Grease a 9-inch baking pan with cooking spray or butter and line the bottom with parchment paper.

Combine brown sugar, flour, and cinnamon in a medium bowl for the streusel topping. Cut in cold butter until mixture resembles coarse crumbs. Set aside.

Cream together softened butter and granulated sugar until light and fluffy. Add eggs one at a time, then mix in vanilla extract.

In a separate bowl, whisk together flour, baking powder, baking soda, and salt. Gradually mix this into the butter mixture until just combined.

Finally, alternately add in the sour cream, gently folding until smooth. Avoid overmixing.

Pour the batter into the prepared pan and spread evenly. Sprinkle the streusel topping generously over the batter.

Bake for 35-40 minutes until a toothpick comes out clean and the top is golden brown.

Cool in the pan for 10-15 minutes before transferring to a wire rack. Slice and serve.