Preheat the oven to 350°F / 175°C. Butter and line a 1 lb. loaf pan with parchment paper, to make a hammock for the loaf.
To a large mixing bowl, add in the oil, sugar, pumpkin puree, eggs, sour cream, and vanilla. Whisk vigorously until well combined.
Add in the flour, baking powder, baking soda, and salt. Carefully mix the dry ingredients into the wet ingredients just until no flour streaks remain.
Chocolate Loaf
In a small mixing bowl, mix together the cocoa powder and some freshly brewed espresso until no lumps remain.
Transfer 340g (1 ½ c.) of the pumpkin batter into the bowl with the cocoa powder. Gently fold the chocolate into the pumpkin batter until the mixture is cohesive.
Alternate spreading a thin layer of the pumpkin batter and chocolate batter into the loaf pan. Once all of the batter has been used, sprinkle the top of the loaf with some granulated sugar (optional.)
Bake for 50-60 minutes, or until a toothpick/knife inserted in the center of the loaf comes out clean.
Allow the loaf to fully come to room temperature before slicing. Serve and enjoy!