Preheat the oven to 350°F(175°C). Grease an 8-inch round or square baking pan or line it with parchment paper.
For the topping, in a mixing bowl, stir together the flour, brown sugar, cinnamon, and salt. Add the melted butter and mix with a fork until coarse crumbs form. Set aside.
For the cake, in a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, pumpkin pie spice, and salt.
In the bowl of a stand mixer (or a large bowl), beat the softened butter and brown sugar until light and creamy, about 2 minutes.
Beat in the eggs one at a time, then add the vanilla.
Mix in the pumpkin puree and sour cream until smooth.
Add the dry ingredients and stir just until combined. Avoid overmixing.
Spread the batter evenly into the prepared pan. Sprinkle the streusel evenly over the top.
Bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs.
Let the cake cool in the pan for at least 20 minutes before slicing.