Start by whisking together the flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl. Set it aside for later.
In a larger bowl, mix together the melted butter, sugar, eggs, milk, sour cream, and vanilla until everything is nicely combined.
Add the dry ingredients to the wet mixture and stir until they're all mixed in. Then, gently fold in the chocolate chips with a rubber spatula.
Cover the muffin batter with plastic wrap and let it sit at room temperature for 30 minutes.
Preheat your oven to 350 degrees Fahrenheit. Line two muffin pans with 12 liners, making sure to space them out.
Once the batter has rested, scoop it into the lined muffin cups. Sprinkle some extra mini chocolate chips on top of each muffin for good measure.
Bake the muffins for 20-23 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. If you're using one pan instead of two, add an extra 4-5 minutes to the baking time.
After baking, let the muffins cool in the pan for 10 minutes before transferring them to a wire rack to cool completely.