Place the parmesan cheese into your Thermomix bowl and mix for 10 seconds, speed 9 to finely grate. Transfer to a new bowl and set aside until needed.
In a clean bowl, add the potatoes and milk and cook for 18 minutes, 100 degrees, speed 1.
Check if your potatoes have cooked through, you want them to be soft in the middle. If still a little hard, cook for a further 3 minutes until soft.
Add the butter to your Thermomix bowl and mix for 15 seconds, speed 5 to combine.
Scrape down the sides of your Thermomix bowl and add the grated parmesan cheese and the salt and pepper and mix for a further 6 seconds to combine. Check to see if the potatoes have been completely mashed and mix for a further 5 seconds, speed 5 if there are still lumps. Serve immediately.