Preheat the oven to 350° and line a 9x13 inch baking tray with parchment paper.
In a bowl whisk together the pumpkin, buttermilk, eggs, sugar, vegetable oil and vanilla until well combined.
In another bowl whisk together the flour, baking soda, cinnamon, ginger and pie spice.
In two stages add the dry ingredients to the wet bowl and whisk well after each addition to create a smooth batter.
Pour the pumpkin cake batter into the prepared baking pan and spread out evenly.
Bake in the oven for around 35 minutes or until the top of the cake springs back when gently pushed.
let the cake cool before adding the frosting.
To make the frosting, cream the cream cheese and the butter together at high speed in a mixer.
Add the powdered sugar and vanilla and beat until smooth and creamy.
Spread the frosting over the cool cake and garnish with chopped walnuts or pecans if desired.
Slice into individual squares.