01 - Place the peeled and chunked potatoes in a large pot and cover with cold water by 1 inch. Add 1 teaspoon of kosher salt to the water.
02 - Bring the water to a boil over medium-high heat, then reduce to a gentle simmer. Cook for 15 to 20 minutes until the potatoes are easily pierced with a fork.
03 - Drain the potatoes thoroughly in a colander and return them to the pot. Let them sit over low heat for 1 to 2 minutes to allow any excess moisture to evaporate.
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 mashed potatoes and fold gently until fully melted and evenly incorporated.
06 - Gradually pour in the warmed milk and heavy cream while stirring continuously until the mixture reaches a silky, smooth texture. Add more liquid for a looser consistency if desired.
07 - Season with the remaining kosher salt and freshly ground black pepper to taste. Transfer to a serving bowl and serve hot.