Preheat the oven to 425 degrees. Prepare a 12-well muffin tin by placing cupcake liners in it.
In a large mixing bowl, beat the oil, sugar, eggs, milk, vanilla and sour cream until well combined.
Add in the flour, cocoa powder, baking powder, baking soda and salt and beat until the mixture is smooth.
Fold in about two-thirds of the chocolate chips and spoon the batter into the prepared cupcake liners, filling each all the way to the top of the well.
Sprinkle the leftover chocolate chips on the tops of the muffins, and bake for five minutes. Then, without opening the oven door, lower the temperature to 350 degrees and bake about 15 more minutes or until a toothpick inserted in the centers of the muffins comes out clean.
Let the muffins sit in the tin on the counter for about five minutes before removing them and placing them on a baking rack. Once the muffins are cool, serve them or store in an airtight container.