Potato Purée

1 1/2 lbs. russet potatoes
1 1/2 lbs. Yukon Gold potatoes
3/4 c. whole milk
4 T. butter

Place potatoes, whole and unpeeled in a large pot. Add 2 T. salt and fill the pot with cold water. Bring to a boil over high heat, turn down the heat to low and simmer about 45 minutes, until tender. When potatoes are cooked through, drain them and set them aside to cool for 10 minutes or so, then peel. Heat the milk and butter in a small saucepan or in the microwave. When potatoes have cooled, pass them through a food mill, potato ricer or mash them and put back into the large pot. Slowly add the heated milk and butter, stirring constantly. Season with salt to taste. Potato purée can be placed in a microwavable dish, covered and refrigerated overnight and heated again just before serving.