Peel potatoes and cut into quarters.
Place potatoes in a large stockpot and cover with cold water, add salt (about 1-2 teaspoons) 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.
Drain potatoes and mash with a potato masher or mix in a stand mixer with whisk attachment.
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.
