Preheat oven to 350ºF. Grease a 12-cup muffin pan or line with paper liners.
In a mixing bowl, stir the flour, sugar, cocoa powder, baking powder, baking soda and salt until thoroughly combined.
In another bowl, whisk the milk, egg, melted butter and vanilla until smooth. Add to dry flour mixture and stir until no pockets of flour remain. Do not overmix.
Fold in the chocolate chips. Spoon batter into the prepared muffin tin.
Bake for about 20 minutes or until a toothpick inserted in the center comes out clean. Allow muffins to slightly cool before serving.
Notes
You can store the muffins at room temperature for 3-5 days or in the freezer for up to 3-4 months.