Grease 9 ½ inch bundt pan well, dust with flour and shake out excess.
In a large bowl, whisk together sugar, flour, baking powder, baking soda, salt and spices.
In another large bowl, whisk together oil, buttermilk, vanilla and pumpkin puree. Add eggs one at a time, beating well after each addition.
Combine the pumpkin mixture into the flour mixture using a spatula to fold together until well combined. Then fold in walnuts.Pour batter into prepared bundt pan.
Bake for 60 minutes or until toothpick inserted in the center comes out clean.
Remove from oven and cool for 30 minutes, leaving it in the baking pan.Then invert cake onto a plate and let cool completely.
For the Frosting
Whisk together powdered sugar, cinnamon and whipping cream.
Drizzle over the fully cooled cake and top with walnuts.
Notes
Store leftovers in airtight container in refrigerator.