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 gentle simmer. Cook for 15 to 20 minutes until the potatoes are fork-tender and yield easily when pierced.
03 - Drain the potatoes thoroughly in a colander, then return them to the pot. Let them rest over low heat for 1 to 2 minutes so any residual 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 unsalted butter to the mashed potatoes and fold gently until fully melted and evenly incorporated.
06 - Gradually pour in the warmed whole milk and heavy cream, stirring continuously until the mixture reaches a silky, smooth consistency. Add additional liquid for a looser texture if desired.
07 - Season with the remaining kosher salt and freshly ground black pepper to taste. Adjust seasoning as needed and serve immediately while hot.