Preheat the oven to 425°F (220°C). Grease the muffin pan with non-stick oil spray or line it with paper liners, and set aside.
If using paper liners, lightly grease them as well.
Sift flour, cocoa powder, baking powder, baking soda, and salt together and whisk well in a medium bowl.
Add chocolate chips to the flour mixture, and mix well.
Add yogurt or sour cream, sugar, and oil in a large mixing bowl. Whisk until everything is well combined and the mixture is smooth.
Add milk and vanilla and whisk until everything is well combined and smooth.
Fold dry ingredients into wet ingredients and gently mix until well combined. But do not over mix. The batter will be thick.
Divide the batter evenly between the muffin tins. Fill each muffin tin all the way up to the top.
I recommend using an ice cream scoop for easy and even distribution of the batter.
Bake the muffins for 5 minutes at 425°F (220°C), and then, keeping the muffins in the oven, reduce the oven temperature to 350°F (180°C).
Continue baking for 13-15 minutes or until the cake tester inserted in the center comes out clean.
Because every oven is different, keep an eye on the oven after 12 minutes of baking time after reducing the temperature.
Remove the muffins from the oven and let them cool in the pan for 3-4 minutes.
Then transfer them onto a cooling rack and let them cool completely before serving.