Preheat the oven to 190 C. Line 10 muffin moulds with liners.
In a microwave-safe mixing bowl, melt the butter but make sure it doesn't boil or become too hot.
Add the sugar and vanilla, then stir to combine. Add the egg and stir thoroughly until smooth. I prefer using a silicone spatula here.
Now add the yoghurt, stirring again till smooth and the mixture looks thicker.
Sift in the flour, cocoa, baking powder, baking soda and salt (if using).
Fold gently to combine, do not over mix the batter. Add the milk and stir slowly. The batter will be thick, but if it feels very stiff, add a little more milk.
Fold in the chocolate chips, then divide the batter among 9 to 10 muffin moulds. I made 9, but the batter overflowed a bit, so I'd recommend making 10 to be safe, filling each mould about 3/4ths full.
Bake for 5 minutes at 190 C, then lower the temperature and continue baking for 15 minutes, or until the muffins have risen and a toothpick poked in the center comes out clean.
Dot the tops of the hot muffins with a few more chocolate chips for extra deliciousness!
Allow them to cool in the tray for 5 minutes, the remove and place on a wire rack to cool completely, before transferring to an airtight tin. The muffins will stay fresh for 2 to 3 days at room temperature, after which in humid environments, they are best stored in the fridge for up to a week. Keep in mind that at room temperature, the muffins will become quite soft. Happy baking!