Preheat the oven to 425°F (218°C). Spray a 12-count muffin pan with nonstick spray or, as I did, use these cupcake liners. You can also use a 6-count muffin pan and bake in batches. This recipe yields 12–14 muffins.
Then, combine the dry ingredients: In a large mixing bowl, whisk the flour, sugar, cocoa powder, baking soda, a pinch of salt, and chocolate chips and set aside.
Next, combine the wet ingredients: in a separate mixing bowl, add the eggs, sour cream, oil, milk, and vanilla extract. Whisk without overmixing.
Fold in wet into dry ingredients: Pour the egg mixture (wet) into the flour mixture (dry) and fold using a silicone spatula or wooden spoon. The batter is quite thick, so I recommend avoiding using a whisk. I also recommend not overmixing.
Divide the batter between the muffin cups: Spoon the batter into the liners, filling them to the top. This is the moment when you can add the extra chocolate chips to create a rich chocolate topping.
Bake: I recommend baking for 5 minutes at 425°F, then, after the 5 minutes, avoiding opening the oven door, just reduce the oven temperature to 350°F and bake for an additional 12-15 minutes or until a toothpick inserted in the center of the muffin comes out clean. The total time is about 20 minutes. If you decide to make these mini muffins, it is important to adjust the baking time. I suggested baking for 10-12 minutes at 350°F, skipping the high temperatures.
Cool Time: Out of the oven, cool muffins for 10 minutes in the pan, then transfer the muffins to a wire rack until ready to enjoy!
Storage: Any leftover muffins can be stored at room temperature for up to 4-5 days or in the refrigerator for up to a week.