Preheat oven to 180 degrees celsius (fan-forced). Line two large trays with baking paper and set aside.
Insert the butterfly and add the softened butter (chopped), brown sugar and caster sugar. Mix on Speed 4, 30 seconds or until smooth and creamy.
Reduce to Speed 2 and add the eggs, one at a time, through the MC hole.
Add the vanilla extract and return to Speed 4 for a further 10 seconds.
Remove the butterfly and add the plain flour, cocoa powder, salt and bi-carb soda. Mix on Speed 5, 20 seconds.
Add the chocolate chips and mix on Reverse, Speed 4, 10 seconds.
Place the dough into the fridge for 30 minutes to chill (see notes).
Roll heaped teaspoons of the mixture into balls and press down slightly. Place 5 cm apart on the baking trays.
Bake for 8-10 minutes or until just set (see note). Allow to cool for 10 minutes before carefully transferring to a wire rack to cool completely. Store in an airtight container at room temperature for up to a week.