Heat oven to 350 degrees. In a mixing bowl, stir together the flour, spices and salt. Sift into another bowl.
Add the dark brown sugar, white sugar, butter and vanilla extract to a separate large mixing bowl. Beat until light and fluffy.
Beat in the eggs one at a time. Once the eggs are fully incorporated, alternate between adding the flour mixture and buttermilk.
Stir in the pumpkin purée just until combined. The pumpkin pound cake batter is now ready for baking.
Grease and flour a 12-cup bundt pan. The flour makes a big difference in keeping the finished cake from sticking, so don't skip this step. Pour in the batter and smooth on top.
Bake for 55-60 minutes. The cake is ready when a long skewer comes out clean. Allow to cool in the pan for an hour before removing.