Preheat oven to 350 degrees F. Lightly grease the sides and bottom of an 8.5x4-inch loaf pan. Set aside.
In a medium mixing bowl whisk together the sugar, eggs, butter, milk, pumpkin and vanilla until combined.
Place a large fine mesh sieve over the egg mixture. Add 3/4 cup of the flour, baking soda, baking powder, salt and pumpkin pie spice to the sieve and sift into the egg mixture. Use a rubber spatula to gently fold the dry ingredients into the egg mixture. Blend until smooth.
Remove half the batter to another small bowl and add the cocoa powder. Fold to combine. Add the remaining 1/4 cup of flour to the non-chocolate batter. Fold until blended.
Alternate scoops of the two batters into the bottom of the prepared loaf pan, in the following manner: Create two rows of batter, using three alternating scoops in each row. In the left row, make your first scoop pumpkin, then chocolate, then pumpkin. In the other row, start with chocolate, then pumpkin, then chocolate.Repeat with another alternating layer on top. Lightly swirl the combined batters using a large skewer or knife, creating two figure eights, to marble. Top with coarse sugar, if using.
Bake at 350 degrees F for 50 to 55 minutes or until the center is set and firm to the touch and a skewer comes out with no wet batter. Allow the cake to cool for 15 minutes then remove to a wire rack.