Forget heavy cream, cheese, or any other of those traditional mashed potato enhancers. Sour cream adds tang, body, and a rich creaminess that cannot be matched.
In a large pot, bring potatoes to a boil in salted water over high. Reduce to a rapid simmer and cook until potatoes are tender when pierced with a knife, about 15 minutes. Drain and return to pot. Mash until potatoes are fluffy and almost smooth.
In a small saucepan, heat milk and butter over medium-low until butter is melted and mixture is warm. Pour into potatoes and stir to combine. Stir in sour cream and season with salt and pepper.