1cupplus 2 tablespoons buttermilkor substitute 1 cup whole milk with juice of one lemon, divided
Preheat oven to 350°. Prepare one 9-inch loaf pan by buttering and lining with parchment paper.
In a medium bowl, sift or whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, ground cloves, and salt. In a the bowl of an electric mixer fitted with the paddle attachment, cream sugar and butter until smooth. Mix in eggs, one at a time, then add the pumpkin, mixing thoroughly and scraping down the sides of bowl periodically. Add flour mixture and buttermilk, starting with the flour, and alternating until incorporated. Try to end with the flour mixture addition, and be sure not to over mix.
Transfer about 1 cup of the batter into a small bowl. In another small bowl, whisk together the cocoa powder and 2 tablespoons buttermilk. Add the cocoa mixture into the cup of batter and stir until smooth.
Pour half of the pumpkin bread batter into the prepared loaf pan. Add the chocolate batter into the pan and spread evenly. Pour the remaining pumpkin bread batter into the loaf pan and spread over the top. Using a knife, swirl the batters, running the knife from one end of the loaf pan to the other.
Bake for 1 hour and 15 minutes, until a toothpick inserted in the center comes out clean, and the loaf feel springy when you gently press in the middle.