Preheat the oven to 350 degrees.Spray a 9 x 5 inch loaf pan with cooking spray and line the bottom with parchment. In a medium bowl, whisk very well the flour, baking powder, baking soda, cinnamon and salt. Set aside for later.In the bowl of a stand mixer with a paddle attachment or with a hand mixer and a large mixing bowl, cream the butter and sugars together for 3 minutes. Add the eggs, one at a time and continue to beat until combined. If needed, use a rubber spatula at any point to scrape down batter that has come up the sides of the bowl.Mix the vanilla with the buttermilk. With the mixer on low, alternate adding the flour mixture and the buttermilk mixture to the batter in 3 or 4 additions until everything is in and combined. Scoop out one cup of the batter and stir it with the cocoa powder. It will be very thick. Add the pumpkin to the remaining batter. Add a thin layer of the pumpkin batter to the prepared pan then drop spoonfuls of the chocolate batter over the pumpkin. Add a little more of the pumpkin and more spoonfuls of the chocolate. Top with the remaining pumpkin batter and smooth with a rubber spatula. Use a skewer or paring knife to swirl the batters together. Remember that the chocolate batter is much thicker than the pumpkin and will take a lot of effort to break it up. You will feel like you are blending them together too much, but you aren't. Swirl it in circles, cut zig zags and crosses through the batter until you feel very little resistance from the chocolate.Bake the cake for 58 minutes. Remove from the oven and allow to cool completely in the pan before cutting.