Preheat the oven to 350 F and place a rack in the middle of the oven.

In a medium mixing bowl, add in the soft butter, brown sugar, cinnamon, salt, and vanilla. Use a fork to smush it all together until smooth.

Add in the flour and again use the fork to smush the flour into the wet ingredients until no more dry flour is visible. Set aside.

In a medium mixing bowl add in the pumpkin puree, both sugars, eggs, salt, vanilla, buttermilk, pumpkin spice and oil. Whisk to combine.

Sprinkle the flour over the surface of the wet ingredients and then the baking soda on top of the flour.

Gently whisk and fold the wet ingredients into the dry. Be careful not to over mix! Because there is a lot of moisture in the cake it can become tough easily so stop whisking when almost no dry ingredients are still visible.

Line an 8" square baking pan with a rectangle of parchment that reaches from one side to another.

Pour half of the pumpkin batter into the pan and spread it into an even layer. Crumble half of the brown sugar crumb over the pumpkin batter. Spoon the second half of the batter over the crumb and gently spread it into an even layer. Finally sprinkle the second half of the crumb over the top.

Bake the cake on the middle rack in the preheated oven for 25 minutes. After 25 minutes rotate the pan 180 degrees and bake for another 20-25 minutes until the edges are golden brown and a toothpick inserted in the center comes out clean.

Once baked, remove the pan from the oven and set aside to cool for at least 15 minutes.

Use the parchment sling to carefully lift the coffee cake out of the pan. Optionally dust the top with powdered sugar and slice to serve.

Store leftovers in an airtight container at room temperature.