Preheat the oven to 325 degrees Fahrenheit and line an 8x8 baking pan with parchment paper, making sure some hangs over the sides for easy lifting later
For the crumb topping, combine half of the flour with all of the rest of the ingredients (butter, brown sugar, granulated sugar, pumpkin spice, melted butter, and salt). Mix with a fork until combined, then add the rest of the flour and continue mixing until the flour is incorporated and a crumble has formed. Set aside.
Combine the dry ingredients (flour, salt, pumpkin spice, baking powder, and baking soda). Set aside.
Add the sugar and room temp butter to the bowl of a standing mixer fitted with the paddle attachment. Start on low, then increase to medium and cream together until combined. Then add the sour cream, pumpkin puree, eggs, and vanilla. Mix on medium until incorporated, making sure to scrap the sides down with a rubber spatula.
Next, turn it to low and add ⅓ of the flour mixture. When everything is combined, add the next ⅓, and so on. When everything is added, stop the mixer and finish mixing by hand using a spatula.
Add ½ of the batter to the prepared cake pan, spreading it out evenly. Then, add the pumpkin butter on top and spread evenly. Sprinkle a handful or 2 of the crumble topping on top of the pumpkin butter, then add the remaining batter on top of that, spreading it out evenly. Add the rest of the crumble topping, gently pressing it in to the top (not too hard!).
Bake in the preheated oven for about 55 minutes, or until the middle is set. Remove from the oven and let sit in the pan for about 15 minutes. When ready, lift with the parchment paper to a cutting board. Dust the top with powdered sugar before cutting.