Assemble all of the ingredients. Fill a large pot with water and place it on the stovetop without bringing it to a boil yet. If working with whole potatoes rather than pre-cut pieces, divide medium potatoes in half and larger potatoes into thirds, keeping the pieces as uniform as possible to promote even cooking.

Season the water generously with kosher salt and bring it to a boil. Reduce the heat to maintain a simmer, then add the potatoes. Cook until exceptionally tender and easily broken apart when tested with a fork.

Drain the cooked potatoes thoroughly, then transfer them back to the still-hot pot. Allow them to remain there briefly so residual moisture can evaporate.

Combine the milk, heavy cream, and softened unsalted butter in a separate saucepan. Warm the mixture gently without allowing it to reach a boil.

Add the softened cream cheese to the warm milk-and-cream mixture. Stir until the cheese is thoroughly incorporated and the mixture becomes smooth.

Mash the drained potatoes with a potato masher or pass them through a potato ricer, depending on the texture preferred.

Pour the warm dairy mixture into the potatoes gradually, mixing as you go. Continue until the liquid is fully incorporated and the potatoes have a smooth, creamy consistency.

Season the mashed potatoes with kosher salt and freshly ground black pepper to taste, adjusting as necessary. If desired, fold in one or two spoonfuls of sour cream along with a small amount of chopped fresh thyme. Serve warm, optionally finishing each portion with additional butter and freshly ground black pepper.