Preheat oven to 375°F and line a muffin tin with liners.
Chop butter into pieces and microwave in 30-second increments until melted.
In a large bowl, whisk together melted butter, pumpkin puree, brown sugar, granulated sugar, sour cream, vanilla, and eggs until well combined.
In a separate bowl, whisk together flour, cocoa powder, baking soda, and baking powder.
Add dry ingredients to the pumpkin mixture and whisk just until combined — don't overmix.
Fold in chocolate chips.
Add about 3 tbsp. of batter to each liner. Bake 20-22 minutes, baking a second batch for the remaining 6 muffins.
Cool and enjoy!