Place the prepared potatoes directly into the Thermomix mixing bowl. Add the salt and milk and cook for 20 - 25 minutes/100°/reverse/speed 1. The potatoes should be tender and breaking apart.
Once the potatoes are soft and cooked, add the butter (and any other optional additions) if you are including any). Then insert the butterfly whisk and mix for 10 - 15 seconds/speed 3.