Rinse the potatoes under cold water then put them in a pot of salted cold water. Bring to a simmer and cook until very tender, 20-25 minutes

Drain potatoes in a strainer.

In the same saucepan, bring the milk and butter to a simmer, until butter has melted.

Add the potatoes and mash with a potato masher until smooth, adding a little extra milk if needed.

Season with salt and pepper.

Serve in a bowl and add remaining butter just before serving.