Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it with non-stick spray.
In a large mixing bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt until well combined.
In another bowl, mix the melted butter, eggs, vanilla extract, and milk until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few lumps are okay.
Gently fold in the chocolate chips until evenly distributed.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.