Peel potatoes and cut into quarters. Place potatoes in a large stockpot and cover with water, add salt (about 1-2 tsp.) and bring to a boil. Allow to boil for about 20-25 minutes or until the potatoes fall apart when poked with a fork.
Combine milk and butter in a microwave safe bowl and place in the microwave for about 40 seconds or until butter is melted. This makes it so the milk doesn't cool off your potatoes when you add it.
Drain potatoes and mash with a potato masher or mix in a stand mixer with whisk attachment. If you do a stand mixer, make sure to keep it on low to medium speed. Slowly add milk/butter mixture until you reach your desired consistency adding more milk if you want the potatoes to be thinner.
Mash in sour cream and salt & pepper. Serve immediately.
*If you can't serve right away keep potatoes warm in an oven safe dish covered with tin foil and placed in the oven to keep warm at about 300 degrees.