Preheat your oven to 350°F (175°C) and line a standard 12-cup muffin tin with paper liners or generously grease with cooking spray. Position your oven rack in the center position for even baking.
In a medium bowl, whisk together the flour, baking soda, pumpkin pie spice, and salt until well combined.
In a large bowl, whisk together the pumpkin puree, oil, eggs, sugar, milk, and vanilla extract until smooth and well incorporated.
Gently fold the dry ingredients into the wet mixture, stirring just until combined. Do not overmix.
In a separate bowl, combine the cocoa powder, additional sugar, milk, and vanilla extract. Stir until you achieve a smooth, glossy chocolate mixture.
Transfer about 1 cup of the pumpkin batter to the chocolate mixture and stir until fully incorporated. This creates your chocolate batter.
Fill each muffin cup about ⅓ full with the pumpkin batter. Add a tablespoon of chocolate batter to each cup, then top with remaining pumpkin batter until cups are about ¾ full.
Use a toothpick or skewer to gently swirl the batters together with a figure-eight motion. Don't overmix – just 2-3 swirls create the perfect marble pattern.
If desired, sprinkle turbinado sugar and mini chocolate chips over the tops of the muffins.
Bake for 18-22 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. Rotating the pan halfway through ensures even baking.
Allow the muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.