Preheat oven to 350°F. Grease a 9x5 loaf pan.
In a large bowl, combine eggs, sugar, vanilla, oil, and squash puree together. Using a hand mixer (or stand mixer), mix on medium speed until fully incorporated.
In another bowl, whisk together dry ingredients (flour, baking soda, baking powder, salt, cinnamon, and nutmeg).
Working in batches, add half of the flour mixture to the squash mixture, and mix on low until just incorporated. Add in half of the milk, mixing on low, and once again remaining flour and milk.
Transfer half of the batter to a medium bowl, and add in the cocoa powder for the chocolate batter.
To make the marbled pattern, add a few scoops of the squash batter to the greased loaf pan, and then a few scoops of chocolate. Almost like a checkboard pattern. After you have one layer done, use a butter knife to run through the batter. Repeat with remaining pumpkin and chocolate batter. Resist the urge to over swirl. The more you swirl, the less marbled it will look.
Bake for 50-60 minutes, or until a toothpick comes out clean. Remove from oven and allow to cool for 5-10 minutes, and then remove the loaf from the pan to cool completely on a wire rack.
