Preheat oven to 350F (175C) and line a 9×9 baking pan with parchment paper or lightly grease the pan with baking spray.
Combine melted butter and sugars in a large bowl and stir until well-combined
Add eggs, stir well
Stir in pumpkin puree, sour cream, and vanilla extract until well combined. Be sure to scrape down the sides and bottom of the bowl to ensure that all ingredients are well combined.
In a separate, medium-sized bowl, whisk together flour, pumpkin spice, cinnamon, baking powder, baking soda, and salt.
Using a spatula, gradually add dry ingredients and wet and stir gently until just combined — do not over-mix!
Spread batter evenly into prepared 9×9 pan. Set aside while you prepare the streusel topping.
Prepare the crumble by whisking together flour, sugars and cinnamon.
Pour the melted butter into the mixture and, using a fork, stir until crumbles form.
Evenly sprinkle streusel topping over the top of your pumpkin batter.
Transfer pan to oven and bake on 350F (175C) for 30 minutes, or until a toothpick inserted in the center comes out clean.
Allow to cool before adding glaze.
If desired, make glaze by whisking together powdered sugar and milk. Start with 2 teaspoons of milk and add a third if it isn't thin enough to drizzle. Stir in vanilla extract.
Drizzle over cooled pumpkin crumb cake.
Cut and serve. Enjoy!