Place potatoes in a large saucepan and cover with water. Bring to a boil, reduce heat and cook until tender, about 10 minutes.
While potatoes are cooking place buttermilk and milk into a small saucepan and cook over a very low heat until just warm. Be careful not to boil or milk will curdle.
Drain the potatoes, add the warmed milk and mash together to desired consistency. Stir in cheese and salt. Top with the butter and let it melt.