Step 1: Preheat your oven to 350°F (175°C) and grease a 9×5 inch loaf pan or a standard bundt pan thoroughly.
Step 2: In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and pumpkin pie spice.
Step 3: In a large bowl, cream the softened butter with both sugars until the mixture is light and fluffy, about 3 minutes.
Step 4: Beat in the eggs one at a time, followed by the vanilla extract, pumpkin puree, and sour cream until well combined.
Step 5: Gradually stir the dry ingredients into the wet ingredients until just combined. Do not over-mix.
Step 6: Transfer approximately 2 cups of the batter into a separate bowl. Sift the cocoa powder into this portion and add the 2 tablespoons of milk. Stir gently until smooth.
Step 7: Spoon alternating dollops of the pumpkin batter and the chocolate batter into the prepared pan.
Step 8: Use a knife to gently swirl the batters together in a marble pattern.
Step 9: Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Step 10: Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.