Streusel Topping

1. Preheat the oven to 350F/180C and line a 9-inch square baking pan with parchment so that the parchment paper overhangs on the sides, this will help you pull the baked cake out of the pan later. Grease the parchment with non-stick baking spray and set aside.
2. In a medium bowl whisk together the flour, brown sugar, and cinnamon. Pour in melted butter and use a spatula to mix it all together to moisten everything, this will take a couple of minutes, you want a wet sand texture, thick clumps are good. Set aside while you prepare the cake.

Cake

1. In the bowl of an electric stand mixer fitted with a paddle attachment or a large bowl with a handheld electric whisk beat the sugar and butter together for 2-3 minutes, scrape down the bowl after the first minute then continue mixing. You want a light fluffy mixture.
2. Mix in the egg and vanilla scraping down the bowl and making sure the egg is fully combined, there should be no trace of egg yolk.
3. Mix in the pumpkin and sour cream until fully combined, the batter will look curdled that is fine. 
4. Place a sieve over the bowl and sift in the flour, baking soda, baking powder, salt, and pumpkin pie spice. 
5. Gently fold in the dry ingredients until just almost combined. Do not over mix.
6. Pour the batter into the prepared pan, it will be thick so use a spoon or angled spatula to spread it into an even layer. Sprinkle all of the crumble on top into an even layer. Bake for 40-50 minutes, since the cake is covered in a crumble it is best to check by using a toothpick test, insert a toothpick into the center of the cake, moist crumbs are okay but if there is any wet batter the cake will need to cook longer.
7. Let the cake cool on a wire rack for at least 30 minutes before slicing. Best served the day it is made but leftovers will last in an airtight container for up to 3 days.