Preheat oven to 425°F (220°C). Line muffin tins.
In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In a separate bowl, whisk eggs, sour cream, milk, oil, and vanilla.
Pour wet ingredients into dry ingredients and stir just until combined. A few lumps are fine.
Fold in chocolate chips.
Fill muffin cups nearly to the top (about ¾–⅞ full).
Sprinkle extra chocolate chips on top.
Bake at 425°F for 5 minutes, then without opening the oven, reduce temperature to 350°F (175°C) and bake another 12–15 minutes, until a toothpick comes out with a few moist crumbs.
