Preheat oven to 350F. Grease and flour a 9x5-inch baking dish. Set aside.
In a medium bowl, whisk together flour, sugar, baking soda, salt, and spices. Set aside.
In a large bowl, stir together buttermilk, pumpkin puree, and oil until combined.
Stir in eggs and vanilla until incorporated.
Add dry ingredients, and stir until just combined.
Stir in nuts if using.
Pour batter into prepared pan.
Bake for 55-65 minutes, or until a wooden toothpick inserted in the center comes out clean.
Remove from oven, and let cool in pan for 10 minutes.
Remove from pan and finish cooling on wire rack.
The Dairy Swap
Substituting regular milk soured with lemon juice for the specified one cup of buttermilk alters the chemical reaction with the baking soda. Real fermented buttermilk provides the necessary acidity to neutralize the base fully. Without that distinct acidity, the loaf yields a heavy, compact crumb instead of a proper rise.
Gluten Management
Overmixing destroys quick breads instantly. Whisk the dry ingredients thoroughly before hydration to distribute the teaspoon of salt and spices evenly. Add wet components to the bowl and fold strictly until flour streaks vanish. Pumping extra strokes into the batter develops unwanted gluten strands, resulting in a rubbery texture.
Baking and Cooling
Set the oven to 350 degrees Fahrenheit and bake for 55 to 65 minutes. Test doneness by inserting a wooden toothpick into the exact center of the loaf. Let the baked bread rest in the metal pan for precisely 10 minutes before transferring it to a wire rack. Trapped steam will make the bottom crust soggy if left inside the tin too long.