Preheat the oven to 350 degrees. Lightly spray the sides and bottom of an 8½ x 4-inch loaf pan. Set aside.
In a medium mixing bowl, whisk together the sugars, eggs and oil until smooth. Add the pumpkin puree, buttermilk and vanilla to the egg mixture and whisk until combined.
Place a fine mesh sieve / sifter over the bowl with the egg and buttermilk mixture. Add the flour, baking soda, salt and spice to the sieve and sift over the bowl with the egg mixture. Use a rubber spatula to gently fold the dry ingredients into the egg and buttermilk mixture. Stir until smooth.
Pour the batter into the prepared pan. Bake at 350 degrees for 50 to 55 minutes or until the center is set and firm to the touch, and the cake is lightly browned and cracked down the middle.
Remove from the oven and allow the cake to rest for 10 to 15 minutes, then remove the cake from the pan to a wire rack to cool.
ADD THE ICING:
In a medium bowl, whisk together the powdered sugar, vanilla and buttermilk until smooth. Add a little more sugar or buttermilk as needed for desired consistency. Drizzle over the top of the cooled cake, sprinkle with chopped pecans (if desired). Slice and serve. Store leftovers in a sealed container in the refrigerator.