Lightly grease a 9×5 loaf pan with butter or line with parchment paper.
In a stand mixer or bowl, mix melted butter, brown sugar, and regular sugar for five minutes until light and fluffy.
Combine dry ingredients (flour, baking soda, baking powder, pumpkin spice, and salt) well in a medium bowl.
Add eggs, vanilla, pumpkin puree, and sourdough starter to the brown sugar and butter.
Mix until well combined. Fold in dry ingredients a bit at a time (about 1/3 of the flour mixture) until just incorporated.
Pour into a greased pan. Bake for 50-60 minutes. Test bread at 50 minutes with a toothpick or cake tester. If it comes out clean, it is done. If there is still some batter on the toothpick, place it back into the oven to finish baking.
Allow to cool in the pan for one hour.
Notes
Sourdough discard or active sourdough starter can be used for this recipe. If you are making the quick version and want a tangier flavor, use sourdough discard.