Preheat your oven to 350°F (175°C). Grease your baking pan well with butter or nonstick spray and set aside. This helps prevent sticking and ensures the cake comes out cleanly.
In a large bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, salt, and pumpkin pie spice. Make sure everything is evenly combined so the cake will bake uniformly.
In a separate bowl, whisk the pumpkin puree, melted butter, eggs, vanilla extract, and sour cream until smooth. The mixture should be creamy and well-blended with no lumps.
Pour the wet ingredients into the dry ingredients and gently fold together using a spatula or wooden spoon until just combined. Avoid overmixing here, or the cake can become tough. The batter will be thick and slightly lumpy, which is perfect.
In a small bowl, combine the flour, brown sugar, and cinnamon. Cut in the cold cubed butter using a pastry cutter, two forks, or your fingertips until the mixture resembles coarse crumbs. This crumbly topping adds texture and sweetness that makes the cake extra special.
Spread half of the batter evenly into your prepared pan. Sprinkle half of the streusel over the batter. Spread the remaining batter on top, then finish with the remaining streusel. Bake for 40–45 minutes, or until a toothpick inserted into the center comes out clean. Let cool for 15 minutes before slicing.