Preheat the oven to 350°F/175°C and line and grease a 9×9 inch square pan. Combine brown sugar and cinnamon in a small bowl and set aside. This will be the filling.
1/4 cup dark brown sugar, 1/2 tsp cinnamon
In a medium sized bowl, add flour, brown sugar, white sugar, cinnamon, and melted and slightly cooled butter. Using a fork, mix this until combined. Then sprinkle 2 more tablespoons of flour on top and using your hands, crumble the streusel into larger and smaller pieces. You may need 1 more tbsp of flour depending on if the butter has been completely absorbed or not. Then set aside in the fridge when done.
1/4 cup white sugar, 1/4 cup brown sugar, 1/2 tsp cinnamon, 3/4 cups all-purpose flour, 6 tbsp butter
In a medium-sized bowl, add in flour, baking soda, baking powder, cornstarch, cinnamon, nutmeg, allspice, ginger, and salt. Stir together and set aside.
2 cups all-purpose flour, 1 tsp baking soda, 1 tsp baking powder, 1 tbsp cornstarch, 1/4 tsp ginger, 1/8 tsp nutmeg, 1 1/4 tsp cinnamon, 1/2 tsp salt, 1/8 tsp allspice
In a large bowl, add brown sugar and oil and whisk together. Then add in eggs, pumpkin puree, sour cream, and vanilla. Whisk together until well combined.
1/2 cup oil, 3/4 cups dark brown sugar, 1/2 cup sour cream, 1 cup pumpkin puree, 2 eggs, 1 tbsp vanilla
Add the dry ingredients into the wet and using a spatula, gently fold to create a soft cake batter. Do not over-mix! Once the dry ingredient streaks are gone, stop mixing.
Add half of the batter into your prepared pan and spread evenly. Then sprinkle all of the cinnamon sugar topping, and about 4-5 tbsp of the streusel. Add the remaining half of your batter (I find using a cookie scooper works best to not disturb the layer below) and spread evenly, and then the rest of your streusel on top.
Bake the cake for 35-50 minutes, or until a toothpick inserted comes out dry. Mine took 50 minutes. If it is wet, then the cake hasn’t fully baked yet so continue to bake at 5 minute intervals. Allow it to cool completely.
Slice, and enjoy! This coffee cake is pure bliss. Serve it fresh or next day. Store in the fridge for up to 5 days in an airtight container.