Prepare your baking tins by thoroughly greasing them, set aside.
To make the cake
Using a hand mixer or a stand mixer fitted with the paddle attachment, mix oil and sugar.
Add pumpkin puree and sour cream. Mix well.
Add eggs and vanilla extract. Mix well.
In a separate bowl, whisk together dry ingredients (flour, salt, baking powder, baking soda, pumpkin spice).
In 3-4 portions, add the dry ingredients to the wet ingredients, mixing well after each addition.
Set mixture aside while you prepare the filling and topping.
To make the filling
Whisk together flour, cinnamon and brown sugar.
Cut in the softened butter with a fork or your fingers. Work until mixture is crumbly.
Spread half of the cake batter throughout the greased baking tin, then sprinkle all of the filling over it.
Top with the remaining cake batter and spread evenly, completely covering all of the filling with batter.
To make the topping
Using the same bowl that was used for the filling (if desired, since ingredients were the same), whisk together flour, brown sugar and cinnamon.
Cut in softened butter using a fork or your fingers. Pinch mixture together to form some larger pieces (to resemble a streusel topping) and sprinkle over the top of the batter to generously cover the entire pan.
Bake for 35-45 minutes (baking time will depend on your pan), or until edges just begin to brown.
Remove from oven and let cool to room temperature before removing the cakes from the pan. Use a butter knife to loosen the edges if needed.