Preheat the oven to 180c/160c fan/350f/gas mark 4.
Line a 12 hole muffin tin or 2 x 6 hole muffin tins with muffin or cupcake cases.
Add milk to a jug with the vinegar and set aside.
Make up your coffee (you may use orange juice instead) and set aside to cool.
Sieve the flour, cocoa, sugar, baking powder and salt into a large mixing bowl and stir to combine.
Add the chocolate chips.
Pour the coffee (or orange juice into the milk mixture and add the oil.
Pour the wet ingredients into the dry and fold in until just combined. Do not overmix.
Scoop the muffin batter into the muffin cases and bake for 20-25 minutes until a skewer or toothpick comes out clean. A little moisture is ok with chocolate muffins as you don't want them to be dry.
Leave to cool in the tray for 10 minutes, then move to a baking rack to cook.
Enjoy!
Notes
You may use orange juice instead of coffee.
These muffins can be kept in an airtight container for 3-4 days.
These muffins may be frozen for 3-4 months, then defrosted in the freezer overnight.