Preheat the oven to 425ºF and line a 12-cup muffin tin with paper liners.
To a large bowl, sift the flour, cocoa powder, sugar, baking powder, baking soda, and salt. Whisk to combine.
In a medium bowl whisk together the eggs, sour cream, milk, vegetable oil, and vanilla until well combined.
Pour the wet ingredients into the dry and fold together with a spatula until almost combined with a few streaks of flour remaining throughout the bowl. Add 1 ⅔ cups (300g) of the chocolate chips and stir until evenly distributed.
Fill each cupcake well until almost full with batter, and sprinkle with the remaining ⅓ cup (60g) of the chocolate chips.
Bake at 425ºF for 6 minutes then reduce the temperature to 350ºF and continue baking for an additional 14-16 minutes or until the centers are set and a toothpick inserted into the center comes out clean.
Let the muffins cool in pan for a few minutes, then transfer them to a wire rack to finish cooling.
