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 flour, pumpkin pie spice, baking soda, and salt. Set aside.
Cream softened butter and granulated sugar in a large bowl with an electric mixer.
Mix in eggs and vanilla until smooth, then add pumpkin puree and buttermilk and mix until combined.
Add dry ingredients to the pumpkin mixture and mix until just combined.
Pour the batter into the prepared pan and sprinkle the chilled streusel on top.
Bake for 60-70 minutes, or until a toothpick comes out clean. If the top gets too brown, cover loosely with foil.
Let the cake cool in the pan on a wire rack.