Preheat your oven to 400°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease it to prevent sticking.
In a large bowl, whisk together the flour, hot cocoa mix, cocoa powder, sugar, baking soda, and salt.
In a medium bowl, mix the milk, melted butter, egg, and vanilla extract until well combined. Pour the wet mixture into the flour mixture, gently stirring until just combined.
Fold in the chocolate chips.
Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full. Bake for 15-17 minutes, or until a toothpick inserted into the center comes out clean (or with just a few moist crumbs).
Allow the muffins to cool in the muffin pan for about 5 minutes before transferring them to a wire rack.