Bring a large pot of salted water to a rolling boil. Peel and cube the potatoes into 1 1/2-inch pieces.

Add the potatoes to the boiling water and cook for about 10 to 15 minutes, or until they are very tender and fall apart when pierced with a fork.

While the potatoes are cooking, gently warm the milk and butter in a small saucepan until the butter melts. Do not allow it to boil.

Once the potatoes are cooked, drain them well and return them to the pot or place them in a large bowl suitable for mashing.

Mash the potatoes using a potato masher or hand mixer until smooth. Gradually add the warm milk and butter mixture, continuing to mash until well incorporated.

Stir in the buttermilk and season with salt and black pepper to taste. Continue mashing until the potatoes reach a creamy consistency.

Serve the mashed potatoes hot. If needed, keep them warm over a pot of simmering water, or reheat gently, adding additional warm milk if they become too thick.