Preheat oven to 180 degrees Celsius (fan-forced).
Line a 12-hole muffin tray with paper cases and set aside until needed.
Sift the plain flour and bicarbonate soda into a large bowl.
Add the caster sugar, cocoa powder, salt and chocolate chips and stir together.
Create a well in the centre of the dry mixture.
In a separate bowl, lightly beat the eggs before adding the sour cream, vegetable oil, milk and vanilla extract and whisk together until well combined.
Pour the liquid ingredients into the well in the dry ingredients, and very gently fold the mixture together. The batter should be slightly lumpy; do not over-mix.
Spoon the mixture into the paper cases, filling ⅔ to the top.
Bake for 20-25 minutes, use a skewer to check at 20 minutes - see notes.
Allow to cool in the baking tray for 5 minutes before transferring to a wire rack to cool completely.