Preheat the oven to 220°C/200°C fan/425°F/gas mark 7. Line a 12 hole muffin tin with 8 muffin cases. (Or you can just grease the tin if you have a good non-stick one).
Sift together the flour, cocoa powder, baking powder, bicarbonate of soda and salt in a large bowl.
In a separate jug whisk together the milk, yogurt, oil, sugar, vinegar and vanilla extract until smooth.
Pour the wet ingredints into the dry and stir with a balloon whisk until no dry streaks remain, a few small lumps is fine. Be careful not to over-beat the batter.
Gently fold in the chocolate chips.
Divide the batter between the muffin cases, filling them right to the top. An ice cream scoop is handy for this.
Bake for five minutes then turn the oven down to 180°C/160°C fan/350°F/gas mark 4. Bake for a further 18-20 minutes, until a skewer inserted into the centre comes out clean (avoiding any chocolate chips).
Allow the muffins to cool in the tin for 5 minutes, then transfer them to a wire rack and leave to cool completely. Once cold store in an airtight container.