Preheat your oven to 350°F (175°C). Grease and line your rectangular loaf pan with parchment paper, leaving overhang for easy removal.
In a mixing bowl, whisk together pumpkin puree, granulated sugar, brown sugar, vegetable oil, eggs, and vanilla extract until smooth.
In another bowl, mix dry ingredients for pumpkin batter: all-purpose flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
Combine dry ingredients into pumpkin mixture, folding until just combined. Set aside.
In a separate bowl, whisk dry ingredients for chocolate batter: all-purpose flour, cocoa powder, baking powder, baking soda, and salt.
Add buttermilk to chocolate dry ingredients, mix until smooth chocolate batter forms.
Spoon dollops of pumpkin batter and chocolate batter into pan. Gently swirl batters together using a spatula for the marbled effect.
Bake for 45-50 minutes until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then lift it out using parchment paper overhang. Cool completely on a wire rack.