Cook potatoes in salted water until tender (easily pierced with a fork). About 7-10 minutes. Drain and place in bowl of a mixer fitted with a paddle attachment.
Mix on low speed until most of the lumps are gone.
Add butter. Mix on low speed until incorporated and lumps are gone.
Mix in sour cream, salt and pepper.
With mixer on low speed, slowly add milk until you get the consistancy that you desire, then increase mixer speed to high and whip until smooth and fluffy - a couple of minutes or so. Check seasoning and serve.