Preheat oven to 350ºF. Prepare an 9x5-inch metal loaf pan by greasing it or lining it with parchment paper (allowing an overhang on sides).
In a large bowl, whisk together flour, granulated sugar, baking soda, baking powder, salt and spices.
In a separate bowl, whisk together pumpkin puree, brown sugar, buttermilk, oil and egg.
Using your largest rubber spatula, scrape the pumpkin mixture into the flour mixture and gently fold it in until just combined. A few floury streaks are fine - do not overmix.
Scrape batter into prepared loaf pan and smooth out the top. Bake in centre of oven for 55-65 minutes, until internal temperature reaches 190ºF when tested in several spots in the loaf (or top springs back when gently pressed in the middle/tester comes out clean - but a thermometer is most reliable).
NOTE: A longer, narrower loaf pan takes longer to bake because the batter is deeper and there is less surface area directly exposed to heat. Add up to 10 minutes as needed, checking frequently for doneness with a thermometer.