Preheat the oven to 425 F, and position a rack in the center of the oven. Line a standard-sized muffin pan with 14 paper liners, and lightly spray the paper liners with non-stick baking spray.
In a bowl, whisk together the sour cream, milk, oil, sugar, eggs and vanilla until smooth.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, salt and 3/4 cup of the chocolate chips. Add the dry ingredients to the wet ingredients, and use a spatula to stir the batter together just until moistened, being careful not to over-mix.
Divide the batter between the muffin cups, filling them full. Sprinkle the top of the batter with the remaining 1/4 cup chocolate chips.
When you put the muffins in the oven, immediately turn the heat down from 425 to 375. Bake for about 18-20 minutes, just until a toothpick comes out clean or with moist crumbs clinging to it. Be careful not to over-bake the muffins, or they can dry out.
Cool in the pan for 5 minutes, then transfer to a cooling rack to cool for about 10 more minutes. Enjoy the muffins warm.