In a small bowl, mix together sugar and cinnamon. Set aside.
!to make the cake:
Preheat the oven to 350 degrees F.
Grease and flour a 13" x 9" baking pan. Set aside.
In a large mixing bowl, whisk together the dry ingredients: flour, baking powder, baking soda, brown sugar, granulated sugar, salt and pumpkin spice.
In another bowl, whisk together pumpkin puree, eggs, vanilla, oil and buttermilk.
Add the wet ingredients to the dry ingredients and whisk together until smooth. Whisk for about a minute (I use a whisk but you can use a hand-held mixer).
Pour the batter into prepared pan. Tap the pan against your counter to release any air bubbles.
Sprinkle the cinnamon sugar all over the surface of the cake batter. Sprinkle with nuts.
Bake the cake for 32 to 35 minutes OR until a toothpick inserted in few places near the center comes out clean.