Preheat the oven to 400℉. Prepare muffin tin(s) with muffin liners or grease with butter or non-stick spray.

In a heat-safe bowl in the microwave, slowly melt the butter and chocolate chips together in 15-second increments, stirring between each increment until fully melted. Alternatively, you can melt them together in a saucepan over low heat on the stove.

Whisk the brown sugar, white sugar, eggs, vanilla, sour cream, and milk into the melted butter and chocolate mixture until smooth.

In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Add the combined dry ingredients and chocolate chips to the combined wet ingredients and whisk just until incorporated. Do not overmix! If overmixed, the muffins will be dense and tough.

Distribute the batter between each muffin cup, filling each about to the top with batter. Bake, one pan at a time in the 400℉ preheated oven for 5 minutes before reducing the oven temperature to 350℉ for another 10-12 minutes or until a toothpick inserted into the center comes out clean. Let cool in the muffin tin for about 10 minutes before transferring the muffins to a wire rack to finish cooling so that they don't sweat.