Preheat your oven to 350°F and line the cups of a standard 12-cup muffin pan with paper liners.
In a medium bowl, whisk together flour, sugar, baking powder, baking soda, salt and spices until evenly blended.
In a large bowl, whisk together pumpkin puree with brown sugar until sugar is dissolved. Whisk in eggs one at a time. Whisk in milk, vanilla and melted butter. Add the flour mixture to the pumpkin mixture and stir together gently until just combined. Do not over-mix. A few small lumps are just fine.
Divide batter evenly among prepared muffin cups, filling them to the rim. Drizzle or dollop a teaspoon of melted chocolate over each cup of batter and swirl it through with a toothpick or a skewer. Bake for 20-25 minutes until golden brown and muffins spring back when pressed gently. Transfer pan to a wire rack and let muffins cool for 5 minutes before transferring individually to the rack to finish cooling.