Start by preheating your oven to 350°F (175°C). Grease your loaf pan thoroughly with oil or non-stick spray, then lightly dust it with flour to prevent sticking.
In a large mixing bowl, whisk together the pumpkin puree, granulated sugar, brown sugar, vegetable oil, eggs, and vanilla extract until the mixture is smooth and well combined.
In a separate bowl, sift together the all-purpose flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
Gradually add the dry ingredients to the wet ingredients, mixing gently until just combined. Be careful not to overmix — a few lumps are okay!
Remove about one-third of the batter into a separate bowl. Stir in the unsweetened cocoa powder and half of the chocolate chips until evenly mixed.
Pour half of the plain pumpkin batter into the prepared loaf pan. Spoon half of the chocolate batter over it. Repeat with the remaining plain and chocolate batter. Use a butter knife or skewer to gently swirl the two batters together, creating a beautiful marble effect.
Sprinkle the remaining semi-sweet chocolate chips over the top of the loaf for extra bursts of chocolate in each slice.
Bake in the preheated oven for 55-65 minutes or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. If the loaf starts browning too quickly, tent it loosely with foil halfway through baking.
Allow the loaf to cool in the pan for about 15 minutes before transferring it to a wire rack to cool completely. Serve plain, or with a pat of butter or cream cheese for extra indulgence.