Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a large mixing bowl, combine the chocolate muffin mix, sour cream, vegetable oil, egg, milk, and vanilla extract.
Stir the mixture gently until just combined. Do not overmix; some lumps are okay.
Fold in the mini chocolate chips evenly throughout the batter.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 16 to 18 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.