01 - Place the peeled and chunked potatoes in a large pot. Cover with cold water by 1 inch and add 1 teaspoon of kosher salt.
02 - Bring the pot to a boil over medium-high heat, then reduce to a simmer. Cook until the potatoes are fork-tender, approximately 15 to 20 minutes.
03 - Drain the potatoes thoroughly and return them to the pot. Let them sit over low heat for 1 to 2 minutes so excess moisture evaporates.
04 - Using a potato masher, ricer, or food mill, mash the potatoes until completely smooth and free of lumps.
05 - Add the cubed butter to the potatoes and fold gently until fully melted and evenly incorporated.
06 - Pour in the warm milk and heavy cream gradually, stirring continuously until the mixture is silky and smooth. Add more liquid for a looser texture if desired.
07 - Season with the remaining kosher salt and freshly ground black pepper to taste. Serve immediately while hot.