01 - Place the peeled potato chunks in a large pot and cover with cold, generously salted water by at least 2 inches.
02 - Bring the water to a rolling boil over high heat, then reduce to a gentle simmer. Cook uncovered for 15 to 20 minutes, or until the potatoes yield easily when pierced with a fork.
03 - Drain the potatoes thoroughly in a colander, then return them to the still-warm pot to allow residual heat to evaporate excess moisture.
04 - Add the butter to the hot potatoes and let it sit for 1 to 2 minutes, allowing it to melt completely and coat the potatoes evenly.
05 - Using a potato masher or potato ricer, work the potatoes until completely smooth and free of lumps. For the silkiest results, pass the potatoes through a ricer twice.
06 - Gradually fold in the warmed whole milk and heavy cream in a slow, steady stream, stirring gently until the mixture becomes light, airy, and luxuriously creamy.
07 - Season with salt and freshly ground black pepper to taste. Fold gently until fully incorporated and the texture is velvety smooth throughout.
08 - Transfer to a warmed serving bowl and serve immediately. Garnish with an extra pat of butter or freshly snipped chives if desired.