Preheat the oven to 350°F (175°C) and grease a 9x9-inch baking pan with butter or non-stick spray.

Prepare the streusel topping by mixing brown sugar, 1/2 cup flour, and cinnamon in a bowl; cut in cold cubed butter using a pastry blender until the mixture resembles coarse crumbs.

In a medium mixing bowl, whisk together 2 cups flour, baking soda, baking powder, salt, and pumpkin pie spice.

In a large bowl or stand mixer, cream together the softened butter and granulated sugar until light and fluffy.

Add eggs one at a time to the butter mixture, beating well after each addition.

Incorporate the pumpkin puree, sour cream, and vanilla extract until smooth.

Gradually add the dry ingredient mixture to the wet ingredients, stirring until just combined to avoid overworking the gluten.

Spread the batter evenly into the prepared baking pan.

Distribute the streusel topping uniformly over the batter.

Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan on a wire rack for at least 15 minutes before slicing.