Preheat the oven to 425 degrees farenheit.
In a large bowl, combine your butter and white sugar. Stir together till smooth.
Stir in the milk.
Carefully stir in the eggs and vanilla extract, just until combined.
Fold the sour cream into the batter.
In a separate bowl, combine your dry ingredients - flour, cocoa powder, baking powder, baking soda, and salt. Stir together.
Fold the dry ingredients into the wet ingredients carefully with a spoon or rubber spatula. Don't over mix.
Fold in 1 1/2 cups of the chocolate chips.
Scoop the batter into a greased muffin tin (or a tin lined with paper cupcake liners.) Fill them each about 3/4 full.
Sprinkle remaining 1/4 cup of chocolate chips on top of the muffin batter.
Bake the muffins for about 5 minutes.
Then lower the oven temperature to 350 degrees.
Continue baking the muffins for an additional 12-16 minutes until an inserted toothpick comes out mostly clean (a few crumbs are okay)
Remove the tray from the oven and allow to cool in the pan for about 10 minutes. Then remove to a cooling rack to cool completely. (Or enjoy immediately warm - though they may crumble apart a bit).