Melt the dark chocolate with the milk until smooth — a quick 30–60 seconds in the microwave works great, stirring in between.
Spoon the melted chocolate over the muffin batter, then use a skewer to swirl it in. Alternatively, skip the swirl and sprinkle chocolate chips over the top before baking.
Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean.