Preheat the oven to 350°F. Grease a 9x9 inch metal baking pan and line it with parchment paper.
In a medium bowl, whisk together the flour, pumpkin pie spice, baking soda, and salt. Then set aside the flour mixture.
1 3/4 cups (219 g) all purpose flour, spooned and leveled, 1 tbsp pumpkin pie spice, 1 tsp baking soda, 1/2 tsp salt
Add the softened butter and granulated white sugar to a large bowl. Cream together for 2 minutes with an electric mixer on high speed.
1/2 cup (112 g) unsalted butter, softened, 1 1/4 cups (250 g) granulated white sugar
Then add in the eggs and vanilla and mix on medium speed until pale in color and smooth, about 1 minute.
2 eggs, at room temperature, 1 tsp vanilla
Add in the buttermilk and canned pumpkin puree and mix on medium low speed until combined. (The batter will look curdled at this point but don't worry. It will pull together once the dry ingredients are added.) 1 cup (244 g) canned pumpkin puree, 3/4 cup (180 ml) buttermilk, at room temperature
Add the dry ingredients to the pumpkin mixture and mix on medium low speed just until the batter is combined. Scrape the sides of the bowl with a rubber spatula as necessary.
Pour the batter into the prepared baking pan. Sprinkle the chilled streusel on top of the batter, making sure to cover the entire top.
Bake the coffee cake for 60-70 minutes, or until a toothpick comes out clean from the center with only a few moist crumbs. (If the top is getting too brown towards the end of the baking time, loosely cover with aluminum foil.) Let the cake cool in the pan on a wire rack until cool enough to handle.