Preheat oven to 350℉ and add a muffin liner to each muffin well.
Add melted butter, oil, and cocoa powder to a bowl and mix to combine.
Mix in sugar until combined.
Add the eggs, baking soda, and salt. Whisk until well combined, making sure to scrape down the bottom and sides of the bowl.
Mix in the milk, sour cream, and vanilla until fully incorporated.
Add the flour and mix until combined.
Divide the batter evenly between the 12 lined muffin wells.
Bake for 8 minutes, then sprinkle chocolate chips over each of the muffins.
Bake for 10-14 more minutes or until a toothpick inserted into the center comes out clean or a with a few moist crumbs.
Allow to cool in the pan for a few minutes before transferring to a wire rack to cool.
Notes
Room Temperature Ingredients - having the ingredients at room temperature allows everything to mix and emulsify well, resulting in a smooth and cohesive batter and a more consistent muffin texture.