Preheat the oven to 180°C/350°F (160°C fan)and line a 12-hole muffin tin with cases.
Add the brown sugar, vegetable oil, milk, egg and vanilla extract to the Thermomix bowl. Mix for 10 sec/speed 5 until combined. Scrape down the sides of the Thermomix bowl.
Add the flour and mix for 12 sec/dough function. Add the chocolate chips and use a spatula to fold them gently into the batter, stirring in any loose bits of flour. Avoid overmixing, as the muffins will be less light and fluffy.
Spoon the batter into the cases and bake in the oven for 15-20 minutes until a toothpick inserted into the centre comes out clean.
Transfer the muffins to a baking rack to cool down.
Notes
Storage: Store the muffins in an airtight container in the pantry for up to 3 days.To freeze: Wrap the muffins well in cling film, pop them in a freezer container and store them in the freezer for up to 3 months.