Preheat the oven to 375°F.

Line a 12-cavity muffin pan with muffin liners.

In a large mixing bowl, cream together the butter, granulated sugar, brown sugar, and vanilla extract with a hand mixer until smooth and combined.

Add the eggs and beat for a minute longer, until the mixture is pale and fluffy. Add the sour cream and milk. Beat until fully incorporated.

Add the flour, baking powder, baking soda, cocoa powder, and salt to the bowl. Beat on medium-low speed until just combined. Be careful not to over-mix.

Add the chocolate chips to the batter. Fold with a rubber spatula to evenly distribute.

Spoon an even amount of batter into each muffin liner.

Bake the muffins on middle rack of oven until a toothpick inserted in the centers comes out clean, 15-25 minutes.

Check to see that muffins are done. Remove from oven or add time as needed.

Let muffins cool in the pan for 5 minutes, then remove from the pan and allow to cool completely on a wire rack. Store leftovers at room temperature for 2 days or in the refrigerator for 3 days.