Preheat the oven to 400°F/200°C with a rack in the lower middle.
Using a standard 12-well muffin pan, place a muffin liner in each well, or if you don't have muffin liners, grease the inside of the muffin pan wells with vegetable oil or butter.
Melt the butter in a covered medium-sized microwavable bowl. Mine took a minute on high heat and allow it to cool slightly.
Mix the sour cream, vanilla, oil, and milk right into the warmed butter.
In a large mixing bowl, mix the flour, sugar, baking soda, baking powder, and salt, form a well in the middle, and add the sour cream mixture and eggs. Mix well to combine. Note: The mixture will be thick.
Fold in the chocolate chips.
Divide the batter equally among the muffin cups. You'll need to fill them very high to use all of the batter in 12 muffins, which is fine, and they won't spill. If you still have too much batter, start filling another pan.
Bake until muffin tops are golden and just firm, 15 to 20 minutes, rotating muffin tin from front to back halfway through baking time.
Cool muffins in the muffin pan for 5 minutes, then transfer to wire rack and cool another 5 minutes before serving.
Notes
If using a microwave to melt the butter, cover the bowl as it will splatter and make a mess of your oven.
This batter should be thick, not fluid. If the batter is too runny, add flour, a tablespoon at a time, until it becomes scoopable.
If the batter looks dry or too thick, add extra milk a tablespoon at a time until it reaches the proper consistency.
Don’t overmix – This will result in a rubbery or dense muffin instead of light and fluffy.
For extra chocolatey goodness put extra chocolate chips over the top of the batter.
For mini muffins, bake 9-11 minutes, but keep an eye on them while they bake as mini muffins bake fast.