Preheat oven to 350 degrees F. Lightly grease the sides and bottom of an 8 ½ by 4 inch loaf pan. Set aside.
In a medium mixing bowl, whisk together the sugars, eggs and vegetable 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. Blend until smooth.
Pour the batter into the prepared pan and sprinkle with turbinado sugar, if using.
Bake at 350 degrees F 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 and allow the cake to rest for 10 to 15 minutes, then remove to a wire rack to cool. Drizzle with icing if desired.